Syndecan-4 core protein is sufficient for the assembly of focal adhesions and actin stress fibers

The formation of focal adhesions and actin stress fibers on fibronectin is dependent on signaling through beta 1 integrins and the heparan sulfate pro teoglycan syndecan-4, and we have analyzed the requirement of the glycosami moglycan chains of syndecan-4 during these events. Chinese hamster ovary ce lls with mutations in key enzymes of the glycanation process do not synthes ize glycosaminoglycan chains and are unable to assemble actin stress fibers and focal contacts when cultured on fibronectin. Transfection of the mutan t cells with a cDNA that encodes the core protein of chicken syndecan-4 lea ds to the production of unglycanated core protein. The overexpression of sy ndecan-4 core protein in these mutant cells increases cell spreading and is sufficient for these cells to assemble actin stress fibers and focal adhes ions similar to wild-type cells seeded on fibronectin and vitronectin matri ces. Syndecan-4 core protein colocalizes to focal contacts in mutant cells that have been transfected with the syndecan-4 core protein cDNA. These dat a indicate an essential role for the core protein of syndecan-4 in the gene ration of signals leading to actin stress fiber and focal contact assembly.