Characterization of Multiplication-Stimulating Activity (MSA) carrier protein.

A cloned rat liver cell line (BRL-3A) synthesizes and secretes the somatomedin, Multiplication-Stimulating Activity (MSA), in association with its specific carrier protein (MCP). Affinity-purified MCP is a single-chain polypeptide with a molecular weight of 31,500 under non-reducing conditions and 34,000 when fully reduced. The formation of a Mr 42,000 complex following chemical crosslinking of purified MSA (Mr 8700) and MCP (Mr 34,000) suggests that these components bind in a 1:1 molar ratio on the basis of the sum of their combined molecular weights. The amino acid composition and the N-terminal amino acid sequence of MCP were also determined. Polyclonal MCP-antibody preparations were used to determine if MCP could be detected in normal rat sera. MCP could not be detected in adult rat serum, but was present at high concentrations in fetal rat serum. These results suggest that MCP is a fetal somatomedin carrier protein and that MSA-MCP complexes may be important during fetal development. The availability of antibodies directed against a purified somatomedin carrier protein will provide the opportunity to investigate further the role of carrier proteins in the biological activity of the somatomedins.