Biochemical analysis of p120/130: a protein-tyrosine kinase substrate restricted to T and myeloid cells.

T cell activation is mediated by a cascade of intracellular events involving protein-tyrosine kinases and their substrates. p56(lck) and p59(fyn) are protein-tyrosine kinases that associate with CD4/CD8 and the TCRzeta/CD3 complex, respectively. We previously reported the appearance of a protein doublet at 120 and 130 kDa that preferentially associates with p59(fyn) and undergoes tyrosine phosphorylation upon receptor ligation. In this paper, we demonstrate that p120/130 is a novel protein that is restricted in expression to T cells, thymocytes and myeloid cells. Internal peptide sequencing and immunoblotting using an anti-p120/130 antisera showed that p120/130 is a unique protein that is distinct from p130(cas) and p125(cbl). By contrast, p120 and p130 shared similar peptide patterns and are structurally related. Alkaline phosphatase digestion of precipitates showed that they are not related due to phosphorylation. p120/130 was found to associate constitutively with a 55-kDa protein of unknown identity, but which is distinct from p56(lck) and Shc. p120/130 also undergoes a unique kinetics of phosphorylation and associates with the Ag receptor in response to TCR ligation. In keeping with the association with p59(fyn), T cells from p59(fyn)-negative mice exhibit reduced phosphorylation of the protein. p120/130 therefore represents a novel TCR associated intracellular molecule with potential to play a role in T cell signaling.