Distribution of mRNA transcripts and translation activity in skeletal myofibers.

We examine the distribution of gene products in skeletal myofibers, which are highly differentiated multinucleated cells exhibiting a specific cellular architecture. In situ hybridization studies of adult rat myofibers with a single nucleus infected with influenza virus suggested that the viral mRNA products were distributed beneath the sarcolemma around the nucleus of origin. In situ hybridization studies with a poly-T oligonucleotide probe to detect endogenous mRNAs indicated their concentration around the nuclei and distribution beneath the sarcolemma in a cross-striated fashion at the A-I junctions (costamers). Labeling with bromouridine resulted in a similar distribution pattern. The ribosomal distribution pattern indicated concentration around the myonuclei but an intracellular component was also seen. Localization of the translating ribosomes by puromycylation revealed prominent spots perinuclearly and in the core regions of the myofibers. These spots flanked Golgi elements. Our results thus suggest that the total mRNA pool is heavily concentrated within the perinuclear and subsarcolemmal regions. However, the ribosomes and the translational activity did not follow this distribution pattern, so the mRNA transcripts were not restricted to a region beneath the sarcolemma. Furthermore, experiments utilizing green fluorescent protein showed the rapid movement of proteins within the endomembrane system, which thus facilitated proteins to reach their site of function irrespective of the site of synthesis.