Characterization of the 50 kDa protein phosphorylated in concanavalin A-stimulated mouse T cells.

Concanavalin A (Con A) stimulated the phosphorylation of a protein with a molecular mass of 50 kDa and an approximate pI of 4.7 (p50) in mouse spleen T cells. The phosphorylation of p50 was also stimulated by a protein kinase C activator, phorbol myristate acetate (PMA), but not by a calcium ionophore, A23187. This phosphorylation of p50 was limited to serine and threonine residues in both Con A- and PMA-stimulated T cells. Purification and protein sequence analysis of p50 enabled us to identify it as a lymphocyte-specific putative Ca(2+)-binding protein (LSP-1). These results suggest that p50 may have some function, which is regulated through serine/threonine phosphorylation and the intracellular Ca2+ concentration, downstream of the phosphoinositide signaling system in the T cell activation cascade.