Identification of the 70 kD heat shock cognate protein (Hsc70) and α-actinin-1 as novel phosphotyrosine-containing proteins in T lymphocytes

T cell antigen receptor (TCR) ligation results in the tyrosine phosphorylation of numerous intracellular protein substrates, and the identification of these substrates has been a major undertaking by several groups. We have used pervanadate treatment to artificially increase cellular phosphotyrosine levels and immobilized anti-phosphotyrosine monoclonal antibodies to partially purify tyrosine phosphorylated proteins in quantities suitable for amino acid sequencing. This strategy was used to identify three phosphotyrosine containing proteins, with relative molecular masses of 105, 81, and 76 kD by amino acid sequencing. Here we report the identification of pp105 as α-actinin-1, pp81 as the murine equivalent of the HS1 gene product, and pp76 as Hsc70. This is the first report that α-actinin-1 and Hsc70 are targets of activated tyrosine kinases. Furthermore, we show that Hsc70 is tyrosine phosphorylated in response to TCR ligation, which constitutes the first evidence that Hsc70 might be subject to regulation by tyrosine kinase signaling pathways.