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

The murine sex-limited protein (SIp) is a hemolytically nonfunctional homologue of the fourth component of complement (C4). Two cogenic mouse strains, B10.BUA1 (H-2(w16)) and B10.KPB128 (H-2(w19)), 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 SIp. SIp molecules isolated from the plasma or peritoneal macrophage cultures from these strains have an α-chain ~ 2,000 daltons smaller than th α-chain of SIp from H-2(d) or H-2(w7) mice as judged by sodium dodecyl sulfate (NaDodSO₄)/polyacrylamide gel electrophoresis. Expression of this SIp was constitutive, i.e., not regulated by androgen, and is cis-dominant in F₁ hybrid mice. Autolysis of the different relative molecular mass (M(r)) α-chains at the internal thiolester produced similar M(r) amino-terminal fragments and different M(r) carboxy-terminal fragments. Deglycosylation of the α-chains with trifluoromethanesulfonic acid eliminated most, if not all, the M(r) difference. The M(r) difference was also manifested by the intracellular precursors of SIp and could be eliminated by endoglycosidase H (endo H) treatment. The number of oligosaccharides on the SIp α-chain was deduced by limited endo H treatment of SIp 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 C4, and a scheme depicting some of the structural differences among these molecules was developed.