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Huntingtin interacting protein 14 (HIP14), a membrane-bound palmitoyl transferase, palmitoylates a number of neuronal proteins (including Huntingtin) and affects the trafficking, stability, aggregation, and/or functional activity of substrate proteins. HIP14 contains an N-terminal ankyrin repeat domain that may function in its substrate recognition. Sequence analysis suggests that the HIP14 ankyrin repeats share approximately 50% identity with the ankyrin repeats of G9a and G9a-like protein (GLP) histone lysine methyltransferases. The crystal structure of the HIP14 ankyrin repeats reveals a surface aromatic cage, formed by two tryptophans, one tyrosine, and one methionine. The all-hydrophobic cage resembles the tri-methylated lysine binding pocket of the plant homeodomain (PHD) of human BPTF (bromodomain and PHD domain transcription factor) 1. HIP14, a Huntingtin interacting protein 2, is a 633-residue protein, including 7-8 ankyrin repeats in the N-terminal region followed by five predicted transmembrane helices. The protein contains a signature DHHC palmitoyl transferases motif located close to the predicted fourth transmembrane helix 3. Importantly, the ankyrin repeats (a protein-protein interaction domain that may function in substrate recognition) and the DHHC sequence (the hypothetical active site) are both predicted to reside on the cytoplasmic face of the lipid bilayer 4, presumably allowing the substrate recognition and the active site to interact with the same substrate. Posttranslational palmitoylation involves the attachment of the saturated C16 fatty acid palmitate to specific cysteines via a thioester linkage 5-7. HIP14 palmitoylates Huntingtin at cysteine 214 8. Other substrates of HIP14-mediated palmitoylation include SNAP-25 (synaptosome associated protein 25 kDa), PSD-95 (postsynaptic density 95 kDa), GAD-65 (glutamate decarboxylase 65 kDa), and Synaptotagmin I 9. Ankyrin repeats are known to mediate protein-protein interactions 10. Recently we showed that the ankyrin repeat domains of G9a and GLP, two euchromatin associated histone lysine methyltranferases, bind N-terminal histone H3 peptides containing mono- or di-methylated lysine 9 (H3K9me1, me2) via a partial aromatic cage with three tryptophans and one acidic residue 11. Besides the ankyrin repeats of G9a and GLP, other protein domains including the Chromodomain 12, plant homeodomain 1,13-15, and Tudor domain 16, also recognize methylated lysines (Review 17). The common mode of methyl-lysine interactions is via a surface aromatic cage consisting of 2-4 aromatic residues. These aromatic cages are highly selective for methyllysine. In a study of Chromodomain-methyllysine interaction, binding was driven primarily by cation-π interactions (i.e, a methyllysine carries a positive charge), and secondarily by the packing of methyl group(s) against the aromatic ring(s) of the cage, with the hydrophobic effect contributing the least to binding 18. This eliminates the possibility of such cages generically binding hydrophobic residues, and suggests that other methyllysine binding ankyrin repeats could be identified by the presence of a surface aromatic cage.

Huntington and related neurological diseases result from expansion of a polyglutamine (polyQ) tract. The linear lattice model for the structure and binding properties of polyQ proposes that both expanded and normal polyQ tracts in the preaggregation state are random-coil structures but that an expanded polyQ repeat contains a larger number of epitopes recognized by antibodies or other proteins. The crystal structure of polyQ bound to MW1, an antibody against polyQ, reveals that polyQ adopts an extended, coil-like structure. Consistent with the linear lattice model, multimeric MW1 Fvs bind more tightly to longer than to shorter polyQ tracts and, compared with monomeric Fv, bind expanded polyQ repeats with higher apparent affinities. These results suggest a mechanism for the toxicity of expanded polyQ and a strategy to link anti-polyQ compounds to create high-avidity therapeutics.

The circadian input kinase (CikA) is a major element of the pathway that provides environmental information to the circadian clock of the cyanobacterium Synechococcus elongatus. CikA is a polypeptide of 754 residues and has three recognizable domains: GAF, histidine protein kinase, and receiver-like. This latter domain of CikA lacks the conserved phospho-accepting aspartyl residue of bona fide receiver domains and is thus a pseudo-receiver (PsR). Recently, it was shown that the PsR domain (1) attenuates the autokinase activity of CikA, (2) is necessary to localize CikA to the cell pole, and (3) is necessary for the destabilization of CikA in the presence of the quinone analog 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). The solution structure of the PsR domain of CikA, CikAPsR, is presented here. A model of the interaction between the PsR domain and HPK portion of CikA provides a potential explanation for how the PsR domain attenuates the autokinase activity of CikA. Finally, a likely quinone-binding surface on CikAPsR is shown here.

Considerable compositional differences of water, nucleic acids and proteins were observed between benign and malignant breast tissues based on the curve-fitting analysis of the infrared absorption band in the frequency region 3100–3600 cm–1. Compared to normal tissue, the tendency of an increase in the proportions of strongly H-bonded water (ASBW) with biomolecules and a decrease in that of weakly H-bonded water (AWBW) was observed in abnormal tissues. Subsequently, ASBW/AWBW ratios dramatically increased. The proportions of nucleic acid–NH2 groups (ANA) and the proportions of protein N–H groups (APR) in abnormal tissues increased from those of normal tissue. The ratios ANA/APR in fibroadenoma and carcinoma tissues are higher than those in normal and hyperplasia tissues whose ANA/APR ratios are located in the same region. The ANA and ANA/APR in carcinoma tissue are the highest. These differences may be related to the cell proliferation in abnormal tissues and the unregulated growth and dedifferentiation of carcinoma cells.