Alternative mRNA splicing of the intermediate filament protein, synemin, is developmentally regulated in cardiac myocytes

Synemin binds PKA RII via blot overlays and co‐immunoprecipitates with RII from adult rat cardiomyocytes (CM) through an amphipathic helical AKAP domain. We also show colocalization of synemin with RII at Z‐lines and the perinuclear region using immunohistochemistry with adult CM. In isolated rat neonatal CMs, synemin localized to the sarcolemma but not to filamentous, cytoplasmic desmin. There are two known isoforms of synemin in rat (H and M) that differ by 932bp in the C‐terminus. Using antibodies to each isoform, the two differ in their subcellular location: H is at the sarcolemma, while M is at striations. A polyclonal antibody against H recognizes 210kDa and 100kDa bands (SDS‐PAGE weights) via western blots of CM lysates, suggesting a new putative isoform. Studies by other labs demonstrated an L isoform comprising the rod domain (exons 1–3) and a truncated tail (exon 5) in mouse brain, skeletal muscle, and heart that may be expressed in rat neonatal or adult CMs. Using affinity‐tagged constructs, we are isolating the multimolecular protein complexes formed by each isoform. We hypothesize that these isoforms have variable subcellular locations and binding partners and may be temporally regulated. We conclude that synemin isoforms have different functions and target PKA near different substrates within myocytes.