cDNA-derived primary structure of the 25-kDa subunit of canine microsomal signal peptidase complex

We purified the 25-kDa subunit of the pentameric signal peptidase complex (SPC 25) from canine pancreas microsomes and utilized amino acid sequence data from tryptic fragments for its molecular cloning and sequencing. Its primary structure (calculated molecular mass of 25,831 Da) shows the presence of two hydrophobic domains of which one is likely to serve as a transmembrane segment. Consistent with being the only SPC subunit with at least one intrachain disulfide bond, SPC 25 contains 4 Cys residues. SPC 25 does not contain a cleavable signal peptide, and it is proposed that the transmembrane segment doubles as signal sequence. The primary structure of SPC 25 is unrelated to any of the other three SPC subunits (SPC 22/23, SPC 21, and SPC 18) that have so far been molecularly cloned and sequenced. Neither is it similar to any protein in the data bases, except that it is 95% identical to an open reading frame of an incomplete and randomly isolated human cDNA clone.