Characterization of Free α- and β-Chains of Recombinant Macrophage-Stimulating Protein

Human serum macrophage-stimulating protein (MSP) induces motile activity of murine resident peritoneal macrophages and is a growth and motility factor for epithelial cells. It belongs to the plasminogen-related family of kringle proteins, and is secreted as a single-chain, 78-kDa, biologically inactive pro-MSP. Proteolytic cleavage of pro-MSP at a single site yields active MSP, a disulfide-linked αβ-chain heterodimer. However cleavage of recombinant pro-MSP yielded not only the disulfide-linked heterodimer, but also free α- and β-chains, indicating that some of the recombinant molecules lacked an αβ-chain disulfide. We purified the free chains for characterization. The β-chain of MSP has three extra cysteines, Cys527, Cys562, and Cys672, which are not found in the plasminogen β-chain. Disulfide bond analysis showed a Cys527–Cys562, but also a Cys588–Cys672. Coopting Cys588by Cys672prevented the expected formation of a disulfide between α-chain Cys468and β-chain Cys588. Concomitant studies determined structures of oligosaccharides at the three Asn-linked glycosylation sites of MSP. The oligosaccharides at the three Asn loci are heterogeneous; 11 different sugars were identified, all being sialylated fucosyl biantennary structures. We also located the pro-MSP signal peptide cleavage site at Gly18-Gln19and the scissile bond for formation of mature MSP at Arg483–Val484.