Molecular cloning of S-protein, a link between complement, coagulation and cell-substrate adhesion.

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Abstract
CDNA clones coding for human S‐protein have been isolated using monoclonal antibodies to screen a cDNA library in pEX. These clones are shown to be authentic S‐protein clones on the basis of sequence, composition and immunological criteria. The complete open reading frame sequence for S‐protein has been determined and shows it to be a single polypeptide chain of 459 amino acids preceded by a cleaved leader peptide of 19 residues. No evidence was found for polymorphism of S‐protein suggesting that different molecular weight forms arise by proteolytic degradation. Of the first 44 amino‐terminal residues 42 are identical with the so‐called somatomedin B peptide suggesting that S‐protein is the somatomedin B precursor. Striking homology is found in the rest of the sequence with the serum spreading factor, vitronectin, which has also been shown to contain somatomedin B sequences at its amino terminus. We conclude that S‐protein and vitronectin are identical and discuss the relevance of this finding to the coagulation and complement pathways.