Identification of integrin-like matrix receptors with affinity for interstitial collagens

Antibodies to a rat liver membrane glycoprotein with an M(r) of 115,000 (nonreduced) inhibited the attachment of rat hepatocytes and primary rat heart fibroblasts to both collagen and fibronectin. The M(r) 115,000 glycoprotein cross-reacted immunologically with the β₁-chain of the rat hepatocyte fibronectin receptor (HFNR), and the two proteins showed identical peptide maps after proteolytic cleavage. It was concluded that the M(r) 115,000 protein was similar or identical to the β₁ chain of Arg-Gly-Asp (RGD)-directed matrix receptors. Although collagen type I contains several RGD sequences, the attachment of hepatocytes and fibroblasts to collagen type I was not inhibited by the synthetic peptide GRGDTP in concentrations that blocked adhesion to fibronectin. Furthermore, hepatocytes adhered equally well to collagen fragments, generated by cyanogen bromide cleavage, lacking RGD sequences as to fragments containing this sequence. Antibodies to the M(r) 115,000 protein inhibited the adhesion of hepatocytes to both types of collagen fragments. Taken together, these data indicate the presence of collagen receptors that share the β-subunit with the HFNR but that are not directed to RGD sequences. Tentative α-chains of the collagen matrix receptor complex were isolated by immunoprecipitation of surface ¹²⁵I-labeled fibroblast membrane proteins purified by affinity chromatography on immobilized collagen type I. Data are presented indicating that proteins with M(r) around 145,000 and 170,000 (nonreduced) are associated in noncovalently linked complexes with the M(r) 115,000 protein. These complexes have affinity for collagen and thus have properties expected for integrin-like collagen receptors.