Tyrosine phosphorylation of myelin P₀ and its implication in signal transduction

P₀ a major structural protein of peripheral myelin, belongs to the immunoglobulin superfamily. Sequence comparison of P₀ with PZR, a tyrosine phosphatase SHP-2 binding protein we recently cloned, revealed the presence of an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the intracellular portion of the P₀ molecule. To study the role of this putative ITIM in signal transduction, we have expressed P₀ in HT-1080 and 293 cells. Stimulation of the transfected cells with pervanadate, a powerful inhibitor of tyrosine phosphatases, resulted in tyrosine phosphorylation of P₀ and its association with several tyrosine-phosphorylated proteins. Mutation of Y²²⁰ embedded in the ITIM to phenylalanine abolished the tyrosine phosphorylation and the association. Tyrosine phosphorylation of P₀ and its association with other signaling proteins were also observed in pervanadate-treated RN22 Schwannoma cells, which express endogenous P₀. Furthermore, injection of pervanadate induced tyrosine phosphorylation of P₀ in peripheral nerves of newborn but not adult mice. The physiological importance of the ITIM in P₀ is implied by the fact that a naturally occurred P₀ mutant with a disrupted ITIM has a dominant role in causing Dejerine-Scotts syndrome. Taken together, P₀ is phosphorylated on Try220. The presence of an ITIM in P₀ and its ability to mediate protein-protein interaction through tyrosine phosphorylation indicate that P₀ is not merely a structural protein but may also be a crucial player in cell signaling. (C) 2000 Academic Press.