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Supplementary Table 1 from Identification of Hypoxia-Regulated Proteins in Head and Neck Cancer by Proteomic and Tissue Array Profiling

Hypoxia within solid tumors decreases therapeutic efficacy, and identification of hypoxia markers may influence the choice of therapeutic modality. Here, we used a proteomic approach to identify hypoxia-regulated proteins and validated their use as endogenous indicators of tumor hypoxia. Using two-dimensional gel electrophoresis and PowerBlot (antibody-based array), we identified a group of 20 proteins that are increased ≥1.5-fold during hypoxia. The majority of these proteins such as IκB kinase β (IKKβ), MKK3b, highly expressed in cancer (HEC), density-regulated protein 1, P150glued, nuclear transport factor 2, binder of ARL 2, Paxillin, and transcription termination factor I have not been previously reported to be hypoxia inducible. The increase in these proteins under hypoxia was mediated through posttranscriptional mechanisms. We additionally characterized the role of IKKβ, a regulator of the nuclear factor-κB transcription factor, during hypoxia. We demonstrated that IKKβ mediates cell survival during hypoxia and is induced in a variety of squamous cell carcinoma cell lines. Furthermore, we showed that IKKβ expression from tumor specimens correlated with tumor oxygenation in patients with head and neck squamous cell carcinomas. These data suggest that IKKβ is a novel endogenous marker of tumor hypoxia and may represent a new target for anticancer therapy.

Background: The Bruker MALDI Biotyper® method utilizes matrix-assisted laser desorption/ionization time-of-flight MS for the rapid and accurate identification and confirmation of Gram-negative bacteria from select media types. The alternative method was evaluated in a method extension study of AOAC INTERNATIONAL First Action Official MethodSM 2017.09 using nonselective and selective agars to identify Cronobacter spp., Salmonella spp., Campylobacter spp., and select Gram-negative bacteria. Results obtained by the Bruker MALDI Biotyper were compared to the traditional biochemical methods as prescribed in the appropriate reference methods. Methods: Two collaborative studies were organized, one in the United States focusing on Cronobacter spp. and other Gram-negative bacteria and one in Europe focusing on Salmonella spp. and other Gram-negative bacteria. Fourteen collaborators from seven laboratories located within the United States participated in the first collaborative study for Cronobacter spp. Fifteen collaborators from 15 service laboratories located within Europe participated in the second collaborative study for Salmonella spp. For each target organism (either Salmonella spp. or Cronobacter spp.), a total of 24 blind-coded isolates were evaluated. In each set of 24 organisms, there were 16 inclusivity organisms (Cronobacter spp. or Salmonella spp.) and 8 exclusivity organisms (non-Cronobacter spp. and non-Salmonella spp. closely related Gram-negative organisms). For the Campylobacter spp. method extension, 17 collaborators from eight laboratories located within the United States (seven laboratories) and Canada (one laboratory) participated in the collaborative study. A total of 24 blind-coded isolates were evaluated. In each set of 24 organisms, there were 16 inclusivity organisms (Campylobacter spp.) and 8 exclusivity organisms (non-Campylobacter spp. closely related Gram-negative organisms). Results: After testing was completed, the total percentage of correct identifications from each agar type for each strain was determined at a percentage of 100.0% to the genus level for the Cronobacter study and a percentage of 100.0% to the genus level for the Salmonella study. For the Campylobacter method extension, a correct identification and confirmation rate of 100.0% was obtained for the Campylobacter organisms at the species level. For non-Cronobacter, non-Salmonella, and non-Campylobacter organisms, 100.0% were correctly identified. Conclusions: The results indicated that the alternative method produced equivalent results when compared to the confirmatory procedures specified by each reference method. Highlights: The method extension can be modified to include the identification and confirmation of Campylobacter jejuni, Campylobacter coli, and Campylobacter lari.

The mechanism of resistance in carbapenem-resistant Enterobacteriaceae (CRE) has therapeutic implications. We comprehensively characterized emerging mechanisms of resistance in CRE between 2013 and 2016 at a health system in Northern California. A total of 38.7% (24/62) of CRE isolates were carbapenemase gene-positive, comprising 25.0% (6/24) bla(OXA-48 like), 20.8% (5/24) bla(KPC), 20.8% (5/24) bla(NDM), 20.8% (5/24) bla(SME), 8.3% (2/24) bla(IMP), and 4.2% (1/24) bla(VIM). Between carbapenemases and porin loss, the resistance mechanism was identified in 95.2% (59/62) of CRE isolates. Isolates expressing bla(KPC) were 100% susceptible to ceftazidime–avibactam, meropenem–vaborbactam, and imipenem–relebactam; bla(OXA-48 like)–positive isolates were 100% susceptible to ceftazidime–avibactam; and metallo β-lactamase–positive isolates were nearly all nonsusceptible to above antibiotics. Carbapenemase gene-negative CRE were 100% (38/38), 92.1% (35/38), 89.5% (34/38), and 31.6% (12/38) susceptible to ceftazidime–avibactam, meropenem–vaborbactam, imipenem–relebactam, and ceftolozane–tazobactam, respectively. None of the CRE strains were identical by whole genome sequencing. At this health system, CRE were mediated by diverse mechanisms with predictable susceptibility to newer β-lactamase inhibitors.

Carbapenem-resistant Enterobacteriaceae (CRE) are emerging as a major health threat in North America. The mechanism of resistance to carbapenems has therapeutic and public health implications. We comprehensively characterized the underlying mechanisms of carbapenem resistance in CRE isolates recovered between 2013 and 2016 at a health system in Northern California. Genotypic methods were used to detect carbapenemases and plasmid-encoded cephalosporinases, and mass spectrometry was used to quantify relative porin levels for OmpC and OmpF and their analogs. MICs for imipenem-relebactam, meropenem-vaborbactam, ceftazidime-avibactam, and ceftolozane-tazobactam were measured. Whole genome sequencing was used for strain typing. A carbapenemase gene encoding blaOXA-48 like, blaNDM, blaKPC, blaSME, blaIMP, and blaVIM was detected in 38.7% (24/62) of CRE isolates. Porin levels was down at least 2-fold in 91.9% (57/62) of isolates. Including carbapenemase genes and porin loss, the mechanism of resistance was identified in 95.2% (59/62) of CRE isolates. Of the carbapenemase gene-positive isolates, blaKPC -positive isolates were 100% susceptible to ceftazidime-avibactam, meropenem-vaborbactam, and imipenem-relebactam; blaOXA-48 like-positive isolates were 100% susceptible to ceftazidime-avibactam; and blaSME-positive isolates were 100% susceptible to meropenem-vaborbactam and ceftolozane-tazobactam. 100% (38/38), 92.1% (35/38), 89.5% (34/38), and 31.6% (12/38) of carbapenemase gene-negative CRE isolates were susceptible to ceftazidime-avibactam, meropenem-vaborbactam, imipenem-relebactam, and ceftolozane-tazobactam, respectively. None of the CRE strains were genetically identical. In conclusion, at this health system in Silicon Valley, carbapenemase-producing CRE occurred sporadically and were mediated by diverse mechanisms. Nucleic acid testing for blaOXA-48 like, blaNDM, blaKPC, blaIMP, and blaVIM was sufficient to distinguish between carbapenemase-producing and non-producing CRE and accurately predicted susceptibility to ceftazidime-avibactam, meropenem-vaborbactam and imipenem-relebactam.

More than 20 species of Legionella have been identified in relation to human infections. Rapid detection and identification of Legionella isolates is clinically useful to differentiate between infection and contamination and to determine treatment regimens. We explored the use of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) Biotyper system (Bruker Daltonik GmbH, Bremen, Germany) for the identification of Legionella species. The MALDI MS spectra were generated and compared with the Biotyper database, which includes 25 Legionella strains covering 22 species and four Legionella pneumophila serogroups. A total of 83 blind-coded Legionella strains, consisting of 54 reference and 29 clinical strains, were analyzed in the study. Overall, the Biotyper system correctly identified 51 (61.4%) of all strains and isolates to the species level. For species included in the Biotyper database, the method identified 51 (86.4%) strains out of 59 Legionella strains to the correct species level, including 24 (100%) L. pneumophila and 27 (77.1%) non-L. pneumophila strains. The remaining 24 Legionella strains, belonging to species not covered by the Biotyper database, were either identified to the Legionella genus level or had no reliable identification. The Biotyper system produces constant and reproducible MALDI MS spectra for Legionella strains and can be used for rapid and accurate Legionella identification. More Legionella strains, especially the non-L. pneumophila strains, need to be included in the current Biotyper database to cover varieties of Legionella species and to increase identification accuracy.

Purpose To identify a 15-KDa novel hypoxia-induced secreted protein in head and neck squamous cell carcinomas (HNSCC) and to determine its role in malignant progression. Methods We used surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF-MS) and tandem MS to identify a novel hypoxia-induced secreted protein in FaDu cells. We used immunoblots, real-time polymerase chain reaction (PCR), and enzyme-linked immunoabsorbent assay to confirm the hypoxic induction of this secreted protein as galectin-1 in cell lines and xenografts. We stained tumor tissues from 101 HNSCC patients for galectin-1, CA IX (carbonic anhydrase IX, a hypoxia marker) and CD3 (a T-cell marker). Expression of these markers was correlated to each other and to treatment outcomes. Results SELDI-TOF studies yielded a hypoxia-induced peak at 15 kDa that proved to be galectin-1 by MS analysis. Immunoblots and PCR studies confirmed increased galectin-1 expression by hypoxia in several cancer cell lines. Plasma levels of galectin-1 were higher in tumor-bearing severe combined immunodeficiency (SCID) mice breathing 10% O2 compared with mice breathing room air. In HNSCC patients, there was a significant correlation between galectin-1 and CA IX staining (P = .01) and a strong inverse correlation between galectin-1 and CD3 staining (P = .01). Expression of galectin-1 and CD3 were significant predictors for overall survival on multivariate analysis. Conclusion Galectin-1 is a novel hypoxia-regulated protein and a prognostic marker in HNSCC. This study presents a new mechanism on how hypoxia can affect the malignant progression and therapeutic response of solid tumors by regulating the secretion of proteins that modulate immune privilege.

Hypoxia activates all components of the unfolded protein response (UPR), a stress response initiated by the accumulation of unfolded proteins within the endoplasmic reticulum (ER). Our group and others have shown previously that the UPR, a hypoxia-inducible factor–independent signaling pathway, mediates cell survival during hypoxia and is required for tumor growth. Identifying new genes and pathways that are important for survival during ER stress may lead to the discovery of new targets in cancer therapy. Using the set of 4,728 homozygous diploid deletion mutants in budding yeast, Saccharomyces cerevisiae, we did a functional screen for genes that conferred resistance to ER stress–inducing agents. Deletion mutants in 56 genes showed increased sensitivity under ER stress conditions. Besides the classic UPR pathway and genes related to calcium homeostasis, we report that two additional pathways, including the SLT2 mitogen-activated protein kinase (MAPK) pathway and the osmosensing MAPK pathway, were also required for survival during ER stress. We further show that the SLT2 MAPK pathway was activated during ER stress, was responsible for increased resistance to ER stress, and functioned independently of the classic IRE1/HAC1 pathway. We propose that the SLT2 MAPK pathway is an important cell survival signaling pathway during ER stress. This study shows the feasibility of using the yeast deletion pool to identify relevant mammalian orthologues of the UPR. (Mol Cancer Res 2005;3(12):669–77)

Hypoxia within solid tumors decreases therapeutic efficacy, and identification of hypoxia markers may influence the choice of therapeutic modality. Here, we used a proteomic approach to identify hypoxia-regulated proteins and validated their use as endogenous indicators of tumor hypoxia. Using two-dimensional gel electrophoresis and PowerBlot (antibody-based array), we identified a group of 20 proteins that are increased ≥1.5-fold during hypoxia. The majority of these proteins such as IκB kinase β (IKKβ), MKK3b, highly expressed in cancer (HEC), density-regulated protein 1, P150glued, nuclear transport factor 2, binder of ARL 2, Paxillin, and transcription termination factor I have not been previously reported to be hypoxia inducible. The increase in these proteins under hypoxia was mediated through posttranscriptional mechanisms. We additionally characterized the role of IKKβ, a regulator of the nuclear factor-κB transcription factor, during hypoxia. We demonstrated that IKKβ mediates cell survival during hypoxia and is induced in a variety of squamous cell carcinoma cell lines. Furthermore, we showed that IKKβ expression from tumor specimens correlated with tumor oxygenation in patients with head and neck squamous cell carcinomas. These data suggest that IKKβ is a novel endogenous marker of tumor hypoxia and may represent a new target for anticancer therapy.

Purpose: Our study was undertaken to determine the utility of plasma proteomic profiling using surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) mass spectrometry for the detection of head and neck squamous cell carcinomas (HNSCCs). Experimental Design: Pretreatment plasma samples from HNSCC patients or controls without known neoplastic disease were analyzed on the Protein Biology System IIc SELDI-TOF mass spectrometer (Ciphergen Biosystems, Fremont, CA). Proteomic spectra of mass:charge ratio (m/z) were generated by the application of plasma to immobilized metal-affinity-capture (IMAC) ProteinChip arrays activated with copper. A total of 37,356 data points were generated for each sample. A training set of spectra from 56 cancer patients and 52 controls were applied to the “Lasso” technique to identify protein profiles that can distinguish cancer from noncancer, and cross-validation was used to determine test errors in this training set. The discovery pattern was then used to classify a separate masked test set of 57 cancer and 52 controls. In total, we analyzed the proteomic spectra of 113 cancer patients and 104 controls. Results: The Lasso approach identified 65 significant data points for the discrimination of normal from cancer profiles. The discriminatory pattern correctly identified 39 of 57 HNSCC patients and 40 of 52 noncancer controls in the masked test set. These results yielded a sensitivity of 68% and specificity of 73%. Subgroup analyses in the test set of four different demographic factors (age, gender, and cigarette and alcohol use) that can potentially confound the interpretation of the results suggest that this model tended to overpredict cancer in control smokers. Conclusions: Plasma proteomic profiling with SELDI-TOF mass spectrometry provides moderate sensitivity and specificity in discriminating HNSCC. Further improvement and validation of this approach is needed to determine its usefulness in screening for this disease.

Purpose To investigate the correlation between gadolinium contrast-enhancement patterns on T1-weighted magnetic resonance (MR) images and spatial changes in protein expression profiles in human glioblastoma multiforme (GBM) and the use of imaging as a noninvasive technique to evaluate the heterogeneity of solid tumors prior to microarray analysis. Materials and Methods Four patients with MR images and confirmed diagnosis of GBM were enrolled in the study. Intraoperative stereotaxy was used in conjunction with MR images to identify contrast-enhanced (CE) and nonenhanced (NE) regions of the tumor during surgical resection. Total protein was extracted from resected tumor samples using standard techniques and subjected to proteomic analysis using surface enhanced laser desorption/ionization time of flight mass spectrometry (SELDI-TOF-MS). Results We found that protein profiles from CE and NE regions within a given tumor have qualitative and semiquantitative proteomic pattern differences, suggesting an altered gene expression profile that correlates with detectable tissue imaging parameters. We also found that CE regions within the same tumor exhibited distinct differences in protein expression profiles, despite similar histological features. In addition, there were marked similarities in the proteomic patterns among the NE regions across all patients, while the CE regions were distinct, suggesting that the CE regions have complex protein profiles unique to individuals. Conclusion The results demonstrate that major differences in protein expression patterns within a tumor can be correlated to radiographic findings. Image-guided proteomics holds promise for characterizing tissue prior to microarray analysis designed to identify specific diagnostic markers and therapeutic targets within solid tumors. J. Magn. Reson. Imaging 2003;18:530–536. © 2003 Wiley-Liss, Inc.

We describe a new material for the display of biomolecular ligands for use in proteomic analysis. We report here on the construction of the first functionalized polymerized diacetylene thin films (PDTFs) for use in displaying immobilized ligands and their application in mass spectral proteomic analysis. Functionalized polymerized thin film surfaces were constructed with diacetylene-containing biotin lipid monomers designed for the capture of proteins (streptavidin) from a complex cellular lysate and detection with mass spectrometry (MS). These materials serve as a prototype for ligand-based spotted arrays amenable to high throughput screening. Functionalized PDTFs can be easily manufactured for customized microarrays and demonstrate high protein specificity and low nonspecific protein adsorption, and the resulting microarrays constructed from these materials are compatible with several different protein analysis platforms. Our results suggest that these materials have broad potential applications for use in mass spectral-based proteomic analysis.

Glycosylation of ion channel proteins dramatically impacts channel function. Here we characterize the asparagine (N)-linked glycosylation of voltage-gated K+ channel alpha subunits in rat brain and transfected cells. We find that in brain Kv1.1, Kv1.2 and Kv1.4, which have a single consensus glycosylation site in the first extracellular interhelical domain, are N-glycosylated with sialic acid-rich oligosaccharide chains. Kv2.1, which has a consensus site in the second extracellular interhelical domain, is not N-glycosylated. This pattern of glycosylation is consistent between brain and transfected cells, providing compelling support for recent models relating oligosaccharide addition to the location of sites on polytopic membrane proteins. The extent of processing of N-linked chains on Kv1.1 and Kv1.2 but not Kv1.4 channels expressed in transfected cells differs from that seen for native brain channels, reflecting the different efficiencies of transport of K+ channel polypeptides from the endoplasmic reticulum to the Golgi apparatus. These data show that addition of sialic acid-rich N-linked oligosaccharide chains differs among highly related K+ channel alpha subunits, and given the established role of sialic acid in modulating channel function, provide evidence for differential glycosylation contributing to diversity of K+ channel function in mammalian brain.

In the neuroendocrine cell line, PC12, synaptic vesicles can be generated from endosomes by a sorting and vesiculation process that requires the heterotetrameric adaptor protein AP3 and a small molecular weight GTPase of the ADP ribosylation factor (ARF) family. We have now discovered a second pathway that sorts the synaptic vesicle-associated membrane protein (VAMP) into similarly sized vesicles. For this pathway the plasma membrane is the precursor rather than endosomes. Both pathways require cytosol and ATP and are inhibited by GTPγS. The second pathway, however, uses AP2 instead of AP3 and is brefeldin A insensitive. The AP2-dependent pathway is inhibited by depletion of clathrin or by inhibitors of clathrin binding, whereas the AP3 pathway is not. The VAMP-containing, plasma membrane–derived vesicles can be readily separated on sucrose gradients from transferrin (Tf)-containing vesicles generated by incubating Tf-labeled plasma membrane preparations at 37°C. Dynamin- interacting proteins are required for the AP2-mediated vesiculation from the plasma membrane, but not from endosomes. Thus, VAMP is sorted into small vesicles by AP3 and ARF1 at endosomes and by AP2 and clathrin at the plasma membrane.

We have transiently expressed the rat Kv2.1 K+ channel polypeptide (pKv2.1) at high levels by transfection of mammalian COS-1 cells. Kv2.1-transfected cells express a molecular mass of 108 kDa pKv2.1, larger than the size of the core polypeptide (95 kDa) predicted from the deduced primary sequence and of pKv2.1 synthesized in cell free or Xenopus oocyte translation systems. The increased size of pKv2.1 in COS-1 cells is due to a posttranslational modification that occurs early (t1/2 = 5 min) in the biosynthetic transport through the endomembrane system, presumably while the protein resides in the endoplasmic reticulum. The increased size is entirely due to phosphorylation, based on in vivo 32P-labeling and sensitivity to alkaline phosphatase digestion. Immunofluorescent localization of pKv2.1 shows intense surface labeling; no intracellular pools of retained protein are apparent. Immunogold electron microscopy confirms that the expressed polypeptide is found on the cell surface in small clusters or patches of 10-15 gold particles. Cells expressing pKv2.1 exhibit large, voltage-dependent outward currents. The pharmacological properties of the expressed Kv2.1 currents are virtually indistinguishable from those described previously in Xenopus oocytes microinjected with Kv2.1 cRNA, but differences in voltage-dependent properties were observed. High level of expression of functional pKv2.1 in these cells points to the utility of this system for the rapid biochemical, cell biological and electrophysiological analysis of altered forms of pKv2.1, and other members of the K+ channel gene family.

More than 20 species of Legionella have been identified in relation to human infections. Rapid detection and identification of Legionella isolates is clinically useful to differentiate between infection and contamination and to determine treatment regimens. We explored the use of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) Biotyper system (Bruker Daltonik GmbH, Bremen, Germany) for the identification of Legionella species. The MALDI MS spectra were generated and compared with the Biotyper database, which includes 25 Legionella strains covering 22 species and four Legionella pneumophila serogroups. A total of 83 blind-coded Legionella strains, consisting of 54 reference and 29 clinical strains, were analyzed in the study. Overall, the Biotyper system correctly identified 51 (61.4%) of all strains and isolates to the species level. For species included in the Biotyper database, the method identified 51 (86.4%) strains out of 59 Legionella strains to the correct species level, including 24 (100%) L. pneumophila and 27 (77.1%) non-L. pneumophila strains. The remaining 24 Legionella strains, belonging to species not covered by the Biotyper database, were either identified to the Legionella genus level or had no reliable identification. The Biotyper system produces constant and reproducible MALDI MS spectra for Legionella strains and can be used for rapid and accurate Legionella identification. More Legionella strains, especially the non-L. pneumophila strains, need to be included in the current Biotyper database to cover varieties of Legionella species and to increase identification accuracy.

Current methods for identification of yeast from blood cultures may take several days after these microorganisms have been observed by Gram stain smears from positive blood cultures. We explored the use of a matrix-assisted laser desorption ionization (MALDI) Biotyper system in combination with Sepsityper specimen processing and Microflex analysis for improved detection and identification of yeast species directly from positive blood culture specimens demonstrating yeast-like organisms by Gram stain. The limit of detection of yeast species in blood culture medium was determined to be 5.9 × 10 5 CFU, with intra- and interstrain coefficients of variation of 1.8 to 3.6% and 2.9%, respectively. A total of 42 yeast-containing positive blood culture specimens were processed, and the identification results were compared to those obtained by routinely used phenotypic methods. Specimens with discrepant results between the Biotyper and phenotypic methods were identified on the basis of internal transcribed spacer region sequencing. The MALDI Biotyper system correctly identified the 42 specimens to species level, including 28 (66.7%) Candida albicans , 8 (19.0%) Candida parapsilosis , and 5 (11.9%) Candida tropicalis isolates and 1 (2.4%) Cryptococcus neoformans isolate. The entire procedure, from specimen extraction to final result reporting, can be completed within 1 h. Our data indicated that the Sepsityper specimen processing and Microflex analysis by the MALDI Biotyper system provide a rapid and reliable tool for yeast species identification directly from positive blood culture media.

The authentication of foods is an important aspect of quality control and food safety. Honey is one of the most natural and most popular foods in the world. A fast and reliable method to determine the geographical origin of honey was developed based on fingerprinting and barcoding of proteins in honey by using matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI TOF MS) and MALDI Biotyper 1.1 software, respectively. The protein mass spectra of 16 honey samples of known Hawaii origin were obtained and peak information was extracted to generate protein fingerprints. This information was transformed into a database library of spectral barcodes that were used for differentiation of the geographical origin of honeys based on pattern matching. The differentiation ability of the database library of barcodes was validated by comparing the results of replicate assays of 5 of the 16 honey samples of known Hawaii origin obtained directly from the producers. Validation results showed that the protein fingerprints of honeys have better comparability with those honeys in the library known to be from the same region than with those of honey samples from other regions. The protein fingerprints were used to differentiate the geographical origins of commercially purchased honey samples with labels indicating that they were produced in different countries and various states of the USA, including Hawaii. The results showed that the MALDI TOF MS Biotyper system can be a rapid, simple and practical method for determining the geographical origin of honeys sold in commerce.

Motivation: Early cancer detection has always been a major research focus in solid tumor oncology. Early tumor detection can theoretically result in lower stage tumors, more treatable diseases and ultimately higher cure rates with less treatment-related morbidities. Protein mass spectrometry is a potentially powerful tool for early cancer detection. We propose a novel method for sample classification from protein mass spectrometry data. When applied to spectra from both diseased and healthy patients, the ‘peak probability contrast’ technique provides a list of all common peaks among the spectra, their statistical significance and their relative importance in discriminating between the two groups. We illustrate the method on matrix-assisted laser desorption and ionization mass spectrometry data from a study of ovarian cancers. Results: Compared to other statistical approaches for class prediction, the peak probability contrast method performs as well or better than several methods that require the full spectra, rather than just labelled peaks. It is also much more interpretable biologically. The peak probability contrast method is a potentially useful tool for sample classification from protein mass spectrometry data. Supplementary Information: http://www.stat.stanford.edu/~tibs/ppc

The voltage-gated delayed-rectifier-type K+ channel Kv2.1 is expressed in high-density clusters on the soma and proximal dendrites of mammalian central neurons; thus, dynamic regulation of Kv2.1 would be predicted to have an impact on dendritic excitability. Rat brain Kv2.1 polypeptides are phosphorylated extensively, leading to a dramatically increased molecular mass on sodium dodecyl sulfate gels. Phosphoamino acid analysis of Kv2.1 expressed in transfected cells and labeled in vivo with 32P shows that phosphorylation was restricted to serine residues and that a truncation mutant, DeltaC318, which lacks the last 318 amino acids in the cytoplasmic carboxyl terminus, was phosphorylated to a much lesser degree than was wild-type Kv2.1. Whole-cell patch-clamp studies showed that the voltage-dependence of activation of DeltaC318 was shifted to more negative membrane potentials than Kv2.1 without differences in macroscopic kinetics; however, the differences in the voltage-dependence of activation between Kv2.1 and DeltaC318 were eliminated by in vivo intracellular application of alkaline phosphatase, suggesting that these differences were due to differential phosphorylation. Similar analyses of other truncation and point mutants indicated that the phosphorylation sites responsible for the observed differences in voltage-dependent activation lie between amino acids 667 and 853 near the distal end of the Kv2.1 carboxyl terminus. Together, these parallel biochemical and electrophysiological results provide direct evidence that the voltage-dependent activation of the delayed-rectifier K+ channel Kv2. 1 can be modulated by direct phosphorylation of the channel protein; such modulation of Kv2.1 could dynamically regulate dendritic excitability.

To begin to study the molecular bases that determine the selective interaction of the beta-subunits of voltage-gated K+ channels with alpha-subunits observed in situ, we have expressed these polypeptides in transfected mammalian cells. Analysis of the specificity of alpha/bet a-subunit interaction indicates that both the Kvbeta1 and Kvbeta2 beta-subunits display robust and selective interaction with the five members of the Shaker-related (Kv1) alpha-subunit subfamily tested. The interaction of these beta-subunits with Kv1 alpha-subunits does not require the beta-subunit N-terminal domains. Thus, the previously observed failure of N-terminal mutants of Kv beta1 to modulate inactivation kinetics of Kv1 family members is not simply due to a lack of subunit interaction. Interaction of these beta-subunits with members of two other subfamilies (Shab- and Shaw-related) could not be detected. Somewhat surprisingly, a member of the Shal-related subfamily was found to interact with beta-subunits; however, this interaction had biochemical characteristics distinct from the beta-subunit interaction with Kv1 family members. In all cases, Kvbeta1 and Kvbeta2 exhibited indistinguishable alpha-subunit selectivity. These studies point to a selective interaction between K+ channel alpha- and beta-subunits mediated through conserved domains in the respective subunits.