CARNITINE MEDIUM LONG-CHAIN ACYLTRANSFERASE OF MICROSOMES SEEMS TO BETHE PREVIOUSLY CLONED SIMILAR-TO 54 KDA PROTEIN OF UNKNOWN FUNCTION

A microsomal protein having N-terminal amino acid sequence SDVLELTDEN, was initially described as a phosphatidyl inositol-specific phospholi pase Calpha when its cDNA was cloned (Bennett et aL, Nature, 334, 268, 1988). Later, this protein, with an estimated molecular mass of 54 to 60 kDa, was shown to lack the phospholipase activity and instead a pro tein disulfide oxidoreductase and a thiol protease activities were asc ribed to it. Following evidences indicated that the protein in questio n is the carnitine medium/long chain acyltransferase (CPT) of microsom es that was recently purified as a approximately 54 kDa protein (Murth y and Bieber, Protein Exp. Purif. 3,75,1992). First, the N-terminal am ino acids of the microsomal CPT showed 100% homology to the sequence d escribed above. Second, during purification of this CPT, the oxidoredu ctase and the thiol protease activities of the microsomes became separ ated from the CPT and these other activities were not found in the app roximately 900 fold enriched CPT preparations. Third, an antibody to t his protein did not immunoprecipitate oxidoreductase of the solubilize d microsomal extract but precipitated the CPT. This same protein has b een studied by others as the ERp61 (endoplasmic reticulum protein), GR P58 (glucose regulated protein), and HIP-70 (hormone induced protein) but its function was not identified.