CHARACTERIZATION OF 2 HUMAN GENES ENCODING ACYL-COENZYME-A - CHOLESTEROL ACYLTRANSFERASE-RELATED ENZYMES

The enzyme acyl coenzyme A:cholesterol acyltransferase 1 (ACAT1) media tes sterol esterification, a crucial component of intracellular lipid homeostasis. Two enzymes catalyze this activity in Saccharomyces cerev isiae (yeast), and several lines of evidence suggest multigene familie s may also exist in mammals. Using the human ACAT1 sequence to screen data bases of expressed sequence tags, we identified two novel and dis tinct partial human cDNAs. Full-length cDNA clones for these ACAT rela ted gene products (ARGP) 1 and 2 were isolated from a hepatocyte (HepG 2) cDNA library. ARGP1 was expressed in numerous human adult tissues a nd tissue culture cell lines, whereas expression of ARGP2 was more res tricted, In vitro microsomal assays in a yeast strain deleted for both esterification genes and completely deficient in sterol esterificatio n indicated that ARGP2 esterified cholesterol while ARGP1 did not, In contrast to ACAT1 and similar to liver esterification, the activity of ARGP2 was relatively resistant to a histidine active site modifier, A RGP1 is therefore a tissue-specific sterol esterification enzyme which we thus designated ACAT2, We speculate that ARGP1 participates in the coenzyme A-dependent acylation of substrate(s) other than cholesterol , Consistent with this hypothesis, ARGP1, unlike any other member of t his multigene family, possesses a predicted diacylglycerol binding mot if suggesting that it may perform the last acylation in triglyceride b iosynthesis,