Variation in the glycosylation of the murine sex-limited protein: comparison with the fourth component of murine complement.

The murine sex-limited protein (Slp) is a hemolytically nonfunctional homologue of the fourth component of complement (C4). Two congenic mouse strains, B10.BUA1 (H-2w16) and B10.KPB128 (H-2w19), which have been previously shown to share a variant form of C4 (Karp et al., J. Biol. Chem., 257: 7330-7335), were examined and found to also produce a variant form of Slp. Slp molecules isolated from the plasma or peritoneal macrophage cultures from these strains have an alpha-chain approximately 2,000 daltons smaller than the alpha-chain of Slp from H-2d or H-2w7 mice as judged by sodium dodecyl sulfate (NaDodSO4)/polyacrylamide gel electrophoresis. Expression of this Slp was constitutive, i.e., not regulated by androgen, and is cis-dominant in F1 hybrid mice. Autolysis of the different relative molecular mass (Mr) alpha-chains at the internal thiolester produced similar Mr amino-terminal fragments and different Mr carboxy-terminal fragments. Deglycosylation of the alpha-chains with trifluoromethanesulfonic acid eliminated most, if not all, the Mr difference. The Mr difference was also manifested by the intracellular precursors of Slp and could be eliminated by endoglycosidase H (endo H) treatment. The number of oligosaccharides on the Slp alpha-chain was deduced by limited endo H treatment of Slp synthesized in the presence of swainsonine, a plant alkaloid that prevents maturation of complex-type oligosaccharides. This method is a simple way to enumerate the complex-type, N-linked oligosaccharides on glycoproteins. The genetic variation in the glycosylation of Slp was compared with the known variation in glycosylation of C4, and a scheme depicting some of the structural differences among these molecules was developed.