FIBRONECTIN MATRIX AS A SCAFFOLD FOR PROCOLLAGEN PROTEASE BINDING AND COLLAGEN PROCESSING

Abstract

The extracellular matrix (ECM) proteins fibronectin (FN) and type I collagen (collagen I) are co-distributed in many tissues and collagens have been shown to depend on a FN matrix for fibrillogenesis. Despite extensive understanding of supramolecular collagen fiber structure and matrix deposition, the molecular interactions of the proteolytic processing of collagen’s biosynthetic precursor, type I procollagen (procollagen I), have not been elucidated, in particular the role of FN matrix in that process. Microscopic analysis of a fibroblast ECM showed co-localization of procollagen I with FN fibrils and inhibition of FN matrix assembly led to a significant reduction of proteolytic cleavage of procollagen to initiate fibril formation, and a concurrent enrichment of collagens containing one or both propeptides. We examined the role of FN matrix in procollagen processing by the C-propeptide proteinase BMP-1. We found that BMP-1, like procollagen, co-localizes with FN fibrils in the matrix microenvironment. Binding studies with FN fragments identified a binding site in FN’s primary heparin binding domain. In solution, BMP-1-FN interactions and BMP-1 cleavage of procollagen I were both enhanced by the presence of heparin suggesting a role for heparin in complex formation during proteolysis. Indeed, addition of heparin enhanced the rate of procollagen cleavage by matrix-bound BMP-1. Our results show that matrix localization of this proteinase facilitates the initiation of collagen assembly and suggest a model in which FN matrix and associated heparan sulfate act as a scaffold to organize enzyme and substrate for procollagen processing.