Lymphocyte isoforms of mouse p50 LSP1, which are phosphorylated in mitogen-activated T cells, are formed through alternative splicing and phosphorylation.

p50 is phosphorylated in mitogen-stimulated T cells, and translocated from the membrane to the cytosol after activation of protein kinase C. Sequence analysis of p50 revealed that it is identical with LSP1, a putative calcium-binding and actin-binding protein. lymphocyte form of p50 exhibits heterogeneity in the apparent molecular mass on SDS-PAGE, 50 and 52 kDa (pp50 and pp52), and each isoform exhibits heterogeneity in the isoelectric point, when examined by two-dimensional PAGE. When the two molecular mass variants of p50 were dephosphorylated with alkaline phosphatase, both isoforms showed the same apparent molecular mass of 50 kDa on SDS-PAGE, but could be distinguished by their distinct isoelectric points. Dephosphorylated pp50 (p50a) has an acidic pI compared with dephosphorylated pp52 (p50b). Comparison of the peptide maps of purified p50a and p50b on HPLC revealed that the difference was limited to one peptide peak. NH2-terminal sequence and mass spectrometric analyses of these peptides showed that the peptides derived from p50a and p50b had the same NH2-terminal amino acid sequence up to eight residues, but had distinct molecular masses, 5,533.4 and 6,318.6 Da, respectively. These data suggested that pp52 (p50b) is the product of the previously cloned cDNA and the reduction in the molecular mass of the p50a-derived peptide could be explained by deletion of six amino acid residues, EHLIRH or HLIRHQ.(ABSTRACT TRUNCATED AT 250 WORDS)