Characterization of laminin receptor messenger ribonucleic acid and protein expression in mouse spermatogenic cells

A cDNA proposed to encode the mouse laminin receptor (MLR) was isolated from a mouse round spermatid expression library. The cDNA contained complete coding and 3' untranslated regions but was missing the first 42 bases from the 5' untranslated region. Northern blot analysis using a 3' EcoRI fragment of the cDNA identified a 1.2-kb transcript in mouse testes and all somatic tissues tested. Additional transcripts of 1.3 and 0.9 kb were present in round spermatids isolated from mouse testes. Northern blots using ribonuclease (RNase) H-treated poly(A)+ RNA indicated that the difference in the size of 1.3-kb round spermatid transcripts and 1.2-kb transcripts was due to differing poly(A)+ tail lengths. The 0.9-kb round spermatid transcript hybridized to all but the 5' end of the 1.2-kb MLR cDNA, suggesting that an alternate start site is used or that transcript processing occurs in these cells. Immunoblot analysis identified proteins in spermatogenic cells corresponding to the 67-70-kDa MLR and its 43-kDa precursor. In addition, ligand binding studies and affinity chromatography procedures indicated that spermatogenic cell proteins of these sizes bind laminin. However, spermatocytes and spermatids are spatially isolated from laminin in the testes, and MLR may have other functions in these cells.