Isolation, properties, and the complete amino acid sequence of a second form of 60-kDa glycoprotein esterase. Orientation of the 60-kDa proteins in the microsomal membrane.

A second form (form 2) of glycosylated esterase was isolated from liver microsomal membranes and characterized. The subunit molecular weight of form 2 is identical to that of the 60-kDa protein previously reported (Ozols, J. (1987) J. Biol. Chem. 262, 15316-15321). The NH2 terminus of the form 2 enzyme is blocked. Digestion of form 2 with pyroglutamyl aminopeptidase, followed by electroblotting and sequence analysis of the blotted protein, indicated that a pyroglutamyl residue was located at the NH2 terminus. Sequence analysis of the deblocked protein as well as characterization of the peptides obtained from enzymatic and chemical cleavages of the intact protein led to the elucidation of its complete amino acid sequence. The protein is a single polypeptide consisting of 532 residues. Carbohydrate is attached at asparaginyl residue 249. The sequence of form 2 esterase is 50% identical to the sequence of form 1 enzyme. The amino acid sequence of the first 26 residues of form 1 enzyme from human liver microsomes shows that 23 residues are identical to that of rabbit form 1, but only 8 residues that are identical to form 2. Treatment of the forms 1 and 2 isozymes with N-glycosidase F or endo-N-acetylglucosaminidase H resulted in a decrease of their subunit molecular weights, indicating that the carbohydrate attached is of the high mannose type. To determine whether the 60-kDa proteins are located on the cytoplasmic or luminal side of the endoplasmic membrane, microsomes were treated with proteolytic enzymes and the two 60-kDa isozymes were isolated and characterized. Sequence analysis of both proteins indicated that their NH2 termini were unaffected by proteolysis. Form 1 isozyme isolated from trypsin-treated microsomes, however, lacked the COOH-terminal heptapeptide (residues 533-539). These results, in addition to the finding of an N-linked carbohydrate, suggest that the two 60-kDa proteins are oriented on the luminal side of the endoplasmic membrane.