Abstract
Human erythroleukemia cells (K562) adhered rapidly on fibronectin (Fn)- coated growth substratum under serum-free conditions. The adhesion could be quantitatively inhibited by the synthetic peptide Arg-Gly-Asp- Ser (RGDS) and upon hemin-induced differentiation or trypsinization of the cells. Many of the cells also displayed rapid spreading that led to a redistribution of F-actin into spreading edges and in many cells also to a formation of typical actin fibers attaching to the ventral aspect of the cells. The spreading of the cells was inhibited by cytochalasin B but not by microtubule-disrupting drugs, suggesting an active role for the microfilament system in the spreading process. Direct overlay assay of electrophoretically separated polypeptides with 125I-Fn showed that in K562 cells there is a major Mr 190,000 Fn-binding protein that is lost upon differentiation. A similar overlay assay with purified plasma and cellular Fns followed by immunostaining with anti-Fn antibodies revealed a reaction with a similar polypeptide. The binding of Fns on the nitrocellulose sheets could be inhibited and the bound Fn eluted by using the RGDS peptide. From octylglucoside extracts of radioactively surface-labeled cells, distinct Mr 190,000/185,000 membrane glycoproteins bound to Fn-heptapeptide-Sepharose, further suggesting that the Mr 190,000 polypeptide would be the Fn-receptor of the K562 cells.
