CD45 Protein-tyrosine Phosphatase Associates with the WW Domain-containing Protein, CD45AP, through the Transmembrane Region

CD45 is a transmembrane protein-tyrosine phosphatase required for antigen receptor signaling in lymphocytes. CD45 activates the Src family protein-tyrosine kinases, p56lck and p59fyn, by dephosphorylating a negative regulatory tyrosine in the carboxyl terminus. Immunoprecipitation of CD45 precipitates p56lck and CD45AP. Although the function of CD45AP is unknown, it has been proposed to be an adapter between p56lck and CD45. To assess the ability of CD45AP to function as an adapter, we determined the regions required for the interaction with CD45 by expressing chimeric proteins in HeLa cells. CD45AP has a region similar to a potential protein-protein interaction domain, the WW domain. Surprisingly, this domain was not necessary for the association with CD45. Rather, a 40-amino acid sequence encompassing the putative transmembrane domain of CD45AP was sufficient to mediate binding to CD45. Similarly, a 39-amino acid sequence encompassing the CD45 transmembrane region was sufficient to direct the interaction with CD45AP. Expression of p56lck with CD45AP resulted in an interaction that could only be detected by in vitro kinase reaction, suggesting that the association of p56lck and CD45AP is weak. These data support a model in which CD45AP links CD45 with other proteins but not necessarily p56lck.