Effector-induced Syk-mediated phosphorylation in human erythrocytes

Band 3 (AE1), the most prominent polypeptide of the human erythrocyte membrane, becomes heavily tyrosine phosphorylated following treatment of intact cells with protein tyrosine phosphatase inhibitors such as diamide, pervanadate, vanadate, or N-ethylmaleimide (NEM). The mechanism underlying this tyrosine phosphorylation is thought to involve the sequential action of two protein tyrosine kinases, Syk (p72syk) and Lyn (p53/56lyn). While Lyn catalysed phosphorylation appears to be strictly dependent on prior phosphorylation of Tyr8 and 21 of band 3 by Syk, little is known about the mechanism of induction of Syk phosphorylation. Data presented here show that both the fraction of Syk that associates with the membrane and the extent of phosphorylation of band 3 differ in response to the above inhibitors. While diamide and NEM stimulate syk translocation to the membrane during their induction of band 3 tyrosine phosphorylation, pervanadate and vanadate induce no change in kinase distribution. Moreover, diamide and NEM-induced Syk recruitment to the membrane are phosphotyrosine independent and involve their preferential association with Triton X-100-insoluble membrane skeletons. Together these data reveal a complex process controlling the association and catalytic activity of protein tyrosine kinases syk and lyn with the human erythrocyte membrane.