Receptor-mediated endocytosis of vitronectin is regulated by its conformational state

Vitronectin is a structurally labile molecule with a native, non-heparin binding form and a conformationally altered, heparin binding form. To understand the physiological significance of the two conformers of vitronectin, we examined the metabolism of both conformers by cultured human skin fibroblasts. Both native and altered vitronectin bound to confluent fibroblast monolayers. Both conformers of vitronectin competed equally well for the binding of altered vitronectin to the cell layer, suggesting that both conformers bound to the same site in the cell layer. In contrast, 125I-altered vitronectin, but not 125I-native vitronectin, was degraded to trichloroacetic acid-soluble radioactivity by the fibroblast monolayer. Degradation of vitronectin was saturable, sensitive to chloroquine, and occurred intracellularly, suggesting that vitronectin was degraded through a lysosomal pathway. Heparin and thrombospondin inhibited the degradation of altered vitronectin. The degradation of native vitronectin was induced by addition of gamma-thrombin which exposes vitronectin's cryptic heparin-binding domain. These studies suggest that the heparin-binding domain in vitronectin is required for the clearance of vitronectin from the matrix. In addition, these data demonstrate that the conformation of vitronectin regulates its half-life in the matrix. These studies provide the first evidence for a distinct function for the conformers of vitronectin.