The presence of methionine or threonine at position 381 in vitronectin is correlated with proteolytic cleavage at Arginine 379

Vitronectin (serum spreading factor, complement S protein, epibolin) is a glycoprotein that mediates cell adhesion and interacts with components of the complement, coagulation, and fibrinolytic systems. It circulates in plasma as a 75-kDa single chain polypeptide and as a two-chain form consisting of 65- and 10-kDa polypeptides linked by a disulfide bond. An individual may have a predominance of the single chain or the two-chain form inherited as a Mendelian trait or have approximately equal amounts of both forms. Inspection of published cDNA sequences suggests that either methionine or threonine can occur at position 381, which is adjacent to the presumed site of proteolytic cleavage (Arg³⁷⁹-Ala³⁸⁰) that gives rise to the two-chain form. We have determined the presence of the Met³⁸¹ and/or Thr³⁸¹ alleles of the vitronectin gene in 42 individuals by oligonucleotide hybridization to genomic DNA. To determine whether this polymorphism is correlated with the susceptibility to cleavage of the Arg³⁷⁹-Ala³⁸⁰ peptide bond in vivo, we have prepared immunoblots of plasma from the same group of individuals. Nineteen individuals homozygous for the Thr³⁸¹ allele had 17 ± 9% (mean ± S.D.) single chain vitronectin in their plasma samples. Nine individuals homozygous for the Met³⁸¹ allele had 66 ± 4% single chain vitronectin. Fourteen heterozygous individuals had 38 ± 13% single chain vitronectin. The differences in mean values were statistically significant (p < 0.001). These results suggest that the presence of threonine rather than methionine at position 381 of vitronectin increases the susceptibility of the protein to cleavage of the Arg³⁷⁹-Ala³⁸⁰ peptide bond in vivo.