CDNA-derived Primary Structure of the Glycoprotein Component of Canine Microsomal Signal Peptidase Complex

Canine microsomal signal peptidase activity has been shown previously to co-migrate as an apparent complex of six polypeptides with molecular masses of 25, 23, 22, 21, 18, and 12 kDa. The 22- and 23-kDa species are differentially glycosylated forms of the same protein, designated SPC 22/23. The amino acid sequence of SPC 22/23 was deduced from cDNA clones. The protein is synthesized without a cleavable amino-terminal signal sequence and contains a single site for N-linked glycosylation. SPC 22/23 appears to be anchored to the rough endoplasmic reticulum membrane by a single hydrophobic segment near its amino terminus, with the remainder of the protein positioned on the lumenal side of the membrane. The amino acid sequence of SPC 22/23 shares homology with tryptic peptides derived from the hen oviduct signal peptidase glycoprotein, one of two possible proteins required for signal peptide processing in the avian system (Baker, R.K., and Lively, M.O. (1987) Biochemistry 26, 8561-8567). Therefore, the complete amino acid sequence of SPC 22/23 presented in this report corresponds to one of two possible proteins required for signal peptide processing in higher eukaryotic cells.