EXPRESSION OF MOUSE LSP1 S37 ISOFORMS - S37 IS EXPRESSED IN EMBRYONICMESENCHYMAL CELLS

Mouse LSP1 is a 330 amino acid intracellular F-actin binding protein e xpressed in lymphocytes and macrophages but not in non-hematopoietic t issues. A 328 amino acid LSP1-related protein, designated S37, is expr essed in murine bone marrow stromal cells, in fibroblasts, and in a my ocyte cell line. The two proteins differ only at their N termini, the first 23 amino acid residues of LSP1 being replaced by 21 different re sidues in S37. The presence of different amino termini suggests that t he LSP1 and S37 proteins are encoded by transcripts arising through al ternative exon splicing. Here we report the genomic organization of th e Lsp1 gene and show that the distinct N termini of LSP1 and S37 are e ncoded by two alternatively used exons, each containing a translationa l start codon. We also demonstrate that alternative 3' acceptor sites are used in the splicing of exon 5. This results in LSP1 and S37 trans cripts that either do or do not contain 18 bp encoding the 6 amino aci ds HLIRHQ of the acidic domain. Therefore, the Lsp1 gene encodes four protein isoforms: full-length LSP1 and S37 proteins, designated LSP1-I and S37-I and the same proteins without the HLIRHQ sequence, designat ed LSP1-II and S37-II. By in situ hybridization analysis we show that the S37 isoforms are expressed in mesenchymal tissue, but not in adjac ent epithelial tissue, of several developing organs during mouse embry ogenesis. This, together with our finding that S37 is an F-actin bindi ng protein, suggests that S37 is a cytoskeletal protein of mesenchymal cells, which may play a role in mesenchyme-induced epithelial differe ntiation during organogenesis.