CHICKEN LECITHIN-CHOLESTEROL ACYLTRANSFERASE - MOLECULAR CHARACTERIZATION REVEALS UNUSUAL STRUCTURE AND EXPRESSION PATTERN

Rapidly growing oocytes in the laying hen are, in addition to the live r, targets of the so-called ''reverse cholesterol transport'' (RCT) (V ieira, P, M., Vieira, A, V,, Sanders, E, J,, Steyrer, E., Nimpf, J,, a nd Schneider, W, J, (1995) J, Lipid Res, 36, 601-610), pointing to the importance of this process in nonplacental reproduction, We have begu n to delineate the details of this unique trans port pathway branch by molecular characterization of the first nonmammalian lecithin-cholest erol acyltransferase (LCAT), the enzyme that catalyzes an early step i n RCT, The biological significance of the enzyme is under scored by th e high degree of protein sequence identity (73%) maintained from chick en to man, Interestingly, the conservation extends much less to the cy steine residues; in fact, two of the cysteines thought to be important in mammalian enzymes (residues 31 and 184 in man) are absent from the chicken enzyme, providing proof of their dispensability for enzymatic activity, Antibodies prepared against a chicken LCAT fusion protein c ross-react with human LCAT and identify a 64-kDa protein present in en zymatically active fractions obtained by hydrophobic chromatography of chicken se rum, The developmental and tissue distribution pattern of LCAT in females is striking; during embryogenesis and adolescence, LCA T expression is extremely high in liver but undetectable in brain, Upo n onset of laying, however, brain LCAT mRNA increases suddenly and is maintained at levels 5 times higher than in liver, in stark contrast t o most mammals, In adult roosters, the levels of LCAT transcripts in b rain are lower than in liver, Together with the molecular characteriza tion of chicken LCAT, these newly discovered developmental changes and gender differences in its expression establish the avian oocyte/liver system as a powerful model to delineate in vivo regulatory elements o f RCT.