P-0, a major structural protein of peripheral myelin, belongs to the immunoglobulin superfamily. Sequence comparison of P-0 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 P-0 molecule. To study the role of this putative ITIM in signal transduction, we have expressed P-0 in HT-1080 and 293 cells. Stimulation of the transfected cells with pervanadate, a powerful inhibitor of tyrosine phosphatases, resulted in tyrosine phosphorylation of P-0 and its association with several tyrosinephosphorylated proteins. Mutation of Y-220 embedded in the ITIM to phenylalanine abolished the tyrosine phosphorylation and the association. Tyrosine phosphorylation of P-0 and its association with other signaling proteins were also observed in pervanadate-treated RN22 Schwannoma cells, which express endogenous P-0. Furthermore, injection of pervanadate induced tyrosine phosphorylation of P-0 in peripheral nerves of newborn but not adult mice. The physiological importance of the ITIM in P-0 is implied by the fact that a naturally occurred P-0 mutant with a disrupted ITIM has a dominant role in causing Dejerine-Scotts syndrome. Taken together, P-0 is phosphorylated on Try(220). The presence of an ITIM in P-0 and its ability to mediate protein-protein interaction through tyrosine phosphorylation indicate that P-0 is not merely a structural protein but may also be a crucial player in cell signaling.