Localization of 17-kDa myosin light chain isoforms in cultured aortic smooth muscle cells

Smooth muscle myosin II contains two 17-kDa essential light chain isoforms (LC17gi and LC17nm) of which the relative contents differ among myosins, To understand the roles of LC17 isoforms in the functions of myosin, we perfo rmed an immunofluorescence microscopic examination of their localization in primary cultured cells isolated from rat aortic smooth muscle. To identify the isoforms, rabbit polyclonal antibodies were prepared against C-termina l nonapeptides corresponding to either LC17gi or LC17nm from porcine aortic smooth muscle myosin, These isoforms differ in only 5 amino acid residues within the C-terminal 9 residues. These antibodies specifically recognize e ach LC17 isoform on urea-PAGE of total rat aortic cell lysates, Immediately after plating, the smooth muscle cells stained heterogenerously with each antibody, indicating differing contents of LC17 isoforms among cells. On do uble staining 1-2 d cultures with both antibodies, LC17nm was detected diff usely throughout the cytoplasm, whereas LC17gi was concentrated in specific regions such as the cell periphery and the base of cytoplasmic processes. These results support the suggestion that myosin containing LC17gi is essen tial for force-generation by aortic smooth muscle and that myosin containin g LC17nm may play an important role in maintaining smooth muscle tension.