Rabbit liver microsomal endopeptidase with substrate specificity for processing proproteins is structurally related to rat testes metalloendopeptidase 24.15.

The detergent extract of rabbit liver microsomes contains an endopeptidase (MEP) with substrate specificity for peptides containing Arg residues at the P1 and P4 positions in the cleavage site (Kawabata, S., and Davie, E. W. (1992) J. Biol. Chem. 267, 10331-10336). These sequences occur in many proproteins such as the vitamin K-dependent proproteins and prohormones. A cDNA coding for MEP has been obtained from three overlapping clones isolated from two rabbit liver lambda gt10 cDNA libraries. The longest open reading frame of the 3507-base pair cDNA codes for a protein of 704 amino acids, of which 406 residues were confirmed by amino acid sequence analysis. MEP contains a putative active site of -His-Glu-X-X-His-, which is typical of mammalian zinc metallopeptidases. Based on a hydropathy plot, MEP is a hydrophilic protein with no transmembrane domain and no NH2-terminal signal sequence. Amino acid sequence analysis identified Asn at the three potential N-glycosylation sites in the enzyme, indicating that MEP contains no N-linked sugar. MEP is homologous with rat testes metalloendopeptidase 24.15 (60% identity), rat mitochondrial intermediate peptidase (24% identity), Escherichia coli dipeptidyl carboxypeptidase (25% identity), and the open reading frame YCL57w present in yeast chromosome III (35% identity).