Cells interact with extracellular matrix molecules by specific receptors. We have characterized membrane binding molecules for fibronectin and collagen. The biosynthesis, regulation, and function of fibronectin receptors were characterized. The beta subunit of mouse and human receptors were found to be synthesized as smaller precursors that required a lengthy maturation period involving asparagine-linked oligosaccharides. Maturation resulted in an activation of fibronectin-binding function and secretion onto the cell surface. Levels of both alpha and beta subunits of the human fibronectin receptor were regulated by transforming growth factor B, which induced synthesis 20- to 40-fold. Fibronectin receptors on the cell surface were shown to exist in a transmembrane orientation. They are often diffuse on rapidly migrating embryonic cells and avian or human tumor cells, but clustered at adhesion sites of stationary cells. These distributions were shown to be independent of receptor phosphorylation. Roles for the fibronectin receptor were shown in embryonic amphibian gastrulation and human platelet function. Antibodies against the receptor blocked gastrulation and platelet adhesion to fibronectin. The receptor corresponds to platelet glycoprotein Ic-IIa, and is involved in activation-independent attachment of platelets to fibronectin-containing substrates. A protein that was decreased after malignant transformation was characterized biochemically. It is a globular monomer enriched in basic amino acids, with a unique amino-terminal 36-residue sequence containing a novel 7-mer tandem repeat unrelated to other heat shock proteins. Heat shock induces levels of mRNA as detected by in vitro translation and Northern blot analysis. Two putative sequential biosynthetic precursors of this hsp47 protein were. identified, both of which were fully biologically active in binding to a collagenous ligand; this immediate functionality contrasts with the requirement for activation of the fibronectin receptor.
