POSTTRANSLATIONAL MODIFICATIONS OF ALPHA(5)BETA(1) INTEGRIN BY GLYCOSAMINOGLYCAN CHAINS - THE ALPHA(5)BETA(1) INTEGRIN IS A FACULTATIVE PROTEOGLYCAN

Cell-fibronectin interactions, mediated through several different rece ptors, have been implicated in a wide variety of cellular properties. Among the cell surface receptors for fibronectin, integrins are the be st characterized, particularly the prototype alpha(5) beta(1) integrin . Using [I-125]iodine cell surface labeling or metabolic radiolabeling with sodium [S-35]sulfate, we identified alpha(5) beta(1) integrin as the only sulfated integrin among beta(1) integrin heterodimers expres sed by the human melanoma cell line Mel-85. This facultative sulfation was confirmed not only by immunoprecipitation reactions using specifi c monoclonal antibodies but also by fibronectin affinity chromatograph y, two dimensional electrophoresis, and chemical reduction, The covale nt nature of alpha(5) beta(1) integrin sulfation was evidenced by its resistance to treatments with high ionic, chaotrophic, and denaturing agents such as 4 nz NaCl, 4 hn MgCl2, 8 M urea, and 6 ar guanidine HCl . Based on deglycosylation procedures as chemical beta-elimination, pr oteinase K digestion, and susceptibility to glycosaminoglycan lyases ( chondroitinase ABC and heparitinases I and II), it was demonstrated th at the alpha(5) beta(1) heterodimer and alpha(5) and beta(1) integrin subunits were proteoglycans. The importance of alpha(5) beta(1) sulfat ion was strengthened by the finding that this molecule is also sulfate d in MG-63 (human osteosarcoma) and HCT-8 (human colon adenocarcinoma) cells.