Expression of mouse LSP1/S37 isoforms. S37 is expressed in embryonic mesenchymal cells.

Mouse LSP1 is a 330 amino acid intracellular F-actin binding protein expressed in lymphocytes and macrophages but not in non-hematopoietic tissues. A 328 amino acid LSP1-related protein, designated S37, is expressed in murine bone marrow stromal cells, in fibroblasts, and in a myocyte cell line. The two proteins differ only at their N termini, the first 23 amino acid residues of LSP1 being replaced by 21 different residues in S37. The presence of different amino termini suggests that the LSP1 and S37 proteins are encoded by transcripts arising through alternative exon splicing. Here we report the genomic organization of the Lsp1 gene and show that the distinct N termini of LSP1 and S37 are encoded by two alternatively used exons, each containing a translational start codon. We also demonstrate that alternative 3' acceptor sites are used in the splicing of exon 5. This results in LSP1 and S37 transcripts that either do or do not contain 18 bp encoding the 6 amino acids 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, designated LSP1-II and S37-II. By in situ hybridization analysis we show that the S37 isoforms are expressed in mesenchymal tissue, but not in adjacent epithelial tissue, of several developing organs during mouse embryogenesis. This, together with our finding that S37 is an F-actin binding protein, suggests that S37 is a cytoskeletal protein of mesenchymal cells, which may play a role in mesenchyme-induced epithelial differentiation during organogenesis.