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Globally, billions of people burn fuels indoors for cooking and heating, which contributes to millions of chronic illnesses and premature deaths annually. Additionally, residential burning contributes significantly to black carbon emissions, which have the highest global warming impacts after carbon dioxide and methane. In this study, we use Fourier transform infrared spectroscopy (FTIR) to analyze fine-particulate emissions collected on Teflon membrane filters from 15 cookstove types and 5 fuel types. Emissions from three fuel types (charcoal, kerosene, and red oak wood) were found to have enough FTIR spectral response for functional group (FG) analysis. We present distinct spectral profiles for particulate emissions of these three fuel types. We highlight the influential FGs constituting organic carbon (OC) using a multivariate statistical method and show that OC estimates by collocated FTIR and thermal–optical transmittance (TOT) are highly correlated, with a coefficient determination of 82.5 %. As FTIR analysis is fast and non-destructive and provides complementary FG information, the analysis method demonstrated herein can substantially reduce the need for thermal–optical measurements for source emissions. [ABSTRACT FROM AUTHOR]

The [Na.sup.+]:HC[O.sup.-.sub.3] cotransporter NBC1 (SLC4A4, variant A, kidney specific) is located exclusively on the basolateral membrane of epithelial cells, implying that this molecule has acquired specific signals for targeting to the basolateral membrane. A motif with the sequence QQPFLS (positions 1010-1015) in the cytoplasmic tail of NBC1 was recently demonstrated to mediate targeting of NBC1 to the basolateral membrane. Here, we demonstrate that mutating the amino acid F (phenylalanine) or L (leucine) at positions 1013 or 1014 to alanine, respectively, resulted in the retargeting of NBC1 to the apical membrane. Furthermore, mutation of the FL motif to FF showed similar properties as the wild-type; however, mutation of the FL motif to LL showed significant intracellular retention of NBC1. Mutating the amino acids Q-Q-P and S (positions 1010-1011-1012 and 1015) to A-A-A and A, respectively, did not affect the membrane targeting of NBC1. Functional studies in oocytes with microelectrode demonstrated that the apically targeted mutants, as well as basolaterally targeted mutants, are all functional. We propose that the FL motif in the COOH-terminal tail of NBC1 is essential for the targeting of NBC1 to the basolateral membrane but is distinct from the membrane-targeting di-leucine motif identified in other membrane proteins. basolateral membrane; apical membrane; acid-base regulation; kidney; transmembrane domain

The sodium-bicarbonate cotransporter NBC1 is targeted exclusively at the basolateral membrane. Mutagenesis of a dihydrophobic FL motif (residues 1013-1014) in the C-terminal domain disrupts the targeting of NBC1. In the present study, we determined the precise constraints of the FL motif required for basolateral targeting of NBC1 by expressing epitope-tagged wild-type and mutant NBC1 in MDCK cells and RNA-injected Xenopus oocytes and examining their subcellular localization. We assayed the functional activity of the mutants by measuring bicarbonate-induced currents in oocytes. Wild-type NBC1 (containing PFLS) was expressed exclusively on the basolateral membrane in MDCK cells. Reversal of the FL motif (PLFS) had no effect on basolateral targeting or activity. Shifting the FL motif one residue upstream (FLPS) resulted in mistargeting of the apical membrane but the FLPS mutant retained its functional activity in oocytes. Shifting the FL motif one residue downstream resulted in a mutant (PSFL) that did not efficiently translocate to the plasma membrane and was instead colocalized with the ER marker, protein disulfide isomerase (PDI). Analysis of circular dichroism (CD) revealed that a short peptide, 20 amino acid residues, of wild-type NBC1 contained a significant alpha-helical structure, whereas peptides in which the FL motif was reversed or C-terminally shifted were disordered. We therefore propose that the specific orientation and the precise location of the FL motif in the primary sequence of NBC1 are strict requirements for the alpha-helical structure of the C-terminal cytoplasmic domain and for targeting of NBC1 to the basolateral membrane. [ABSTRACT FROM AUTHOR]

The Na(+):HCO(3)(-) cotransporter NBC1 (SLC4A4, variant A, kidney specific) is located exclusively on the basolateral membrane of epithelial cells, implying that this molecule has acquired specific signals for targeting to the basolateral membrane. A motif with the sequence QQPFLS (positions 1010-1015) in the cytoplasmic tail of NBC1 was recently demonstrated to mediate targeting of NBC1 to the basolateral membrane. Here, we demonstrate that mutating the amino acid F (phenylalanine) or L (leucine) at positions 1013 or 1014 to alanine, respectively, resulted in the retargeting of NBC1 to the apical membrane. Furthermore, mutation of the FL motif to FF showed similar properties as the wild-type; however, mutation of the FL motif to LL showed significant intracellular retention of NBC1. Mutating the amino acids Q-Q-P and S (positions 1010-1011-1012 and 1015) to A-A-A and A, respectively, did not affect the membrane targeting of NBC1. Functional studies in oocytes with microelectrode demonstrated that the apically targeted mutants, as well as basolaterally targeted mutants, are all functional. We propose that the FL motif in the COOH-terminal tail of NBC1 is essential for the targeting of NBC1 to the basolateral membrane but is distinct from the membrane-targeting di-leucine motif identified in other membrane proteins.