Amino acid sequence homologies and glycosylation differences between the fourth component of murine complement and sex-limited protein.

Limited primary sequence data have been obtained for all three subunits of the fourth component of murine complement (C4) and its related homologue, the sex-limited protein (Slp). These data show a high degree of NH2-terminal homology between C4 and Slp: four of the six residues identified for the alpha chain, seven of eight for the beta chain, and four of four for the gamma chain. This suggests that apparent molecular weight differences between C4 and Slp subunits are not, as previously suggested, due to a shift in the proteolytic processing sites in the pro-Slp polypeptide molecule. Chemical deglycosylation (apparently complete) of the C4 and Slp alpha chains with trifluoromethanesulfonic acid removes the molecular weight difference between them, suggesting that acquisition of extra glycosylation sites in the latter is responsible for this difference.