Soleus muscle nascent polypeptide chain elongation slows protein synthesis rate during non-weight-bearing activity

Protein synthesis rate of the soleus muscle decreases rapidly during non-weight-bearing activity. We isolated polysomes from 18-h non-weight-bearing soleus muscle to investigate the mechanism of this phenomenon. The distribution of polysomal alpha-actin mRNA and 18S rRNA on sucrose density gradients shows that polysomes shift to larger sizes (more ribosomes per mRNA) during non-weight-bearing activity. Furthermore, RNA is mobilized into the polysome pool of the non-weight-bearing soleus muscle; these data indicate that initiation of protein synthesis is not rate limiting. We explain these results as the slowing of nascent polypeptide chain elongation, such that there is a "traffic jam" of ribosomes on the mRNAs, increasing the number of ribosomes per mRNA while, at the same time, decreasing protein synthesis rate. In support of this hypothesis, myoblasts treated with a low dose of cycloheximide (a specific elongation inhibitor) show a similar shift in polysome size. A numerical model of protein synthesis further shows that elongation is more effective than initiation and termination in affecting protein synthesis and polysome size. We conclude that the non-weight-bearing-induced decrease in postural muscle protein synthesis rate is initially caused by slowing of nascent polypeptide chain elongation.