A systematic study of expression and function of Caenorhabditis elegans genes encoding P-type ATPases in subfamily IV, the group of putative transbilayer amphipath transporters

In eukaryotic organisms, the cytofacial and exofacial monolayers of the plasma membrane and certain other cell membranes exhibit substantial differences with respect to lipid composition. The plasma membrane usually contains all of its phosphatidylserine (PS) and a larger portion of phosphatidylethanolamine in the inner leaflet. Viable cells normally conceal plasma membrane PS, while cells undergoing apoptosis or necrosis expose it on the cell surface. P-type ATPases in subfamily IV are the leading candidate transbilayer amphipath transporters (TATs), the proteins postulated to actively translocate PS to the cytofacial half of the compositionally polarized membranes. TATs of the single cell fungus Saccharomyces cerevisiae are presently the best studied. Several mammalian members of the group are also under investigation. However, the currently available data fail to provide a comprehensive view of tissue-specific expression and developmental function of the subfamily members in a multicellular organism. Here are reported findings of the study aimed to systematically investigate subfamily IV P-type ATPases of the nematode Caenorhabditis elegans.;The C. elegans genome contains six open reading frames that encode P-type ATPases in subfamily IV. All six are transcribed. Expression of a nucleus targeted green fluorescent protein reporter under the control of tat-2, tat-3, tat-4 or tat-5 promoter sequence showed that the former three genes are active in tissue-specific patterns, while the latter gene is transcribed ubiquitously. Phenotypic analysis of nematodes with temporarily, via RNA interference, or permanently, by deletion mutagenesis, altered expression of tat-1 through 5 shows that tat-5 is an essential gene. tat-5(RNAi) animals exhibit morphological defects of the gonad, fertilization deficiencies and embryonic lethality. tat-1 through 4 are nonessential. The tat-3; tat-4 gene pair is nonessential as well. tat-1 seems to be required for exposure of PS on the surface of apoptotic cells in the nematode gonad. tat-2 and tat-4 appear to mediate cholesterol metabolism in the digestive system. No notable phenotype was observed with tat-3 . These findings from C. elegans suggest that tat-1 through 4 are tissue specific genes that regulate membrane structure, and tat-5 is the sole housekeeping TAT.