Alternatively spliced pp52 mRNA in nonlymphoid stromal cells.

Abstract

The 52-kDa phosphoprotein, also reported as lymphocyte-specific gene 1 and WP34, is transcribed as a 1.6-kb mRNA in B lymphocytes, B cell lines, and untransformed T cells. This gene encodes a cytoplasmic and plasma membrane-associated protein that is phosphorylated at a casein kinase II site and reportedly binds calcium. Based on these properties, it has been hypothesized that lymphoid form of the 52-kDa phosphoprotein protein may play a role in lymphocyte signal transduction. We show that alternatively spliced mRNA are expressed from this gene in nonlymphoid cell lines (myocytes, stromal cells, fibroblasts). These cell lines do not express the 1.6-kb lymphoid cell-specific transcript. Instead, mRNA of 2.0 and 2.8 kb are detected in varying abundance. A full-length 2.0-kb cDNA has been cloned and sequenced from the BMS2 stromal cell line by conventional screening and polymerase chain reaction-based methods. This cDNA clone, designated S37, has a single open reading frame encoding a 328 amino acid peptide. The nucleotide sequence of the S37 stromal cell cDNA is identical to that of the lymphocyte derived pp52 cDNA from the 3' poly(A) tail to the codon encoding the amino acid at residue 24. This region of the S37 cDNA clone encodes a protein that is identical to that encoded by the lymphoid pp52 cDNA and includes a casein kinase II phosphorylation site. However, the two clones differ in their 5' nucleotide sequence and their NH3 terminal amino acid sequence. This organization is consistent with alternative exon utilization. These results suggest that tissue-specific control mechanisms are used to generate different forms of lymphoid form of the 52-kDa phosphoprotein mRNA in lymphoid cells versus mesoderm-derived, nonlymphoid cell lineages.