Ligation of CD4 Surface Antigen Induces Rapid Tyrosine Phosphorylation of the Cytoskeletal Protein Ezrin

Ezrin is a cytoskeletal protein which is tyrosine phosphorylated in human T lymphocytes upon stimulation through CD3 antigen (Egerton, M., Burgess, W., Chen, D., Druker, B. J., Bretscher, A., and Samelson, L. A., J. Immunol. 149 , 1847, 1992). We found that tyrosine phosphorylation of ezrin was markedly enhanced by ligation of either CD3 or CD4 antigen and peaked between 1 and 2 min. Furthermore, stimulations through CD4 and CD3 antigens were additive. Using the cell line HUT 78 T transfected with either normal human CD4 or mutated CD4 molecules unable to associate with p56lck tyrosine kinase, we showed that this kinase plays a major role in the tyrosine phosphorylation of ezrin. Moreover, CD45R ligation studies provided evidence that the membrane-associated tyrosine phosphatase CD45 activity regulates ezrin tyrosine phosphorylation. Subcellular fractionation showed that although ezrin is mainly located in the cytosol of T cells, anti-CD4-induced ezrin phosphorylation involved the membrane fraction, with no concomitant translocation of the protein from the cytosol to the membrane.