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.