Transcript-specific mRNA trafficking based on the distribution of coexpressed myosin isoforms.

mRNAs encoding four myosin heavy chain (MHC) isoforms were localized in rat skeletal muscle fibers by in situ hybridization. The ratio of MHC transcript signal in the fiber core compared to the fiber periphery was quantified using image analysis. Two distinct patterns of subcellular localization were observed. Type 1 (beta-cardiac) and type 2A MHC mRNAs were located preferentially in the muscle fiber periphery, while type 2B and type 2X mRNAs were distributed homogeneously across the fiber cross section. Since most normal muscle fibers express only a single MHC isoform, this difference in mRNA distribution could reflect either variation in the localization of the synthetic apparatus across different fiber types or differences in the trafficking of different MHC transcripts. To examine the basis for the observed differential distribution in normal muscles, mRNA distribution was assessed in muscle fibers that coexpressed multiple isoforms of the fast MHCs (i.e. types 2A, 2X and 2B), which occurred either in the combination type 2A/2X or type 2X/2B. The quantitative mRNA distribution seen in muscle fibers expressing a single isoform was not significantly different compared to that observed for mRNAs coexpressed in the same fiber (p > 0.6). Given the size similarity and homology of our riboprobes, these data suggest that their subcellular localization may be determined by relatively small differences in the sequences of the mRNAs, perhaps by differential binding of RNA sequence motifs to cytoskeletal elements.