Nonuniform Rates of Turnover of Myofibrillar Proteins in Rat Diaphragm

In recent years m u c h information has been gained on the s t ructure of the contracti le proteins in muscle and of the myofibril. Relat ively little, however, has been learned concerning the assembly and turnover of this organelle in l iving str iated muscle. The present studies were undertaken to de termine whether in the adul t an ima l the myofibril turns over as a uni t or whether the individual myofibri l lar proteins are synthesized and degraded at different rates. In embryonic muscle, there is s t rong evidence tha t the various contracti le proteins are synthesized at distinct rates (1) on separate polysomes (2, 3) and tha t they even first appea r at distinct times dur ing embryogenesis (3, 4). Previous studies of the turnover of the myofibril in adul t muscle have been inconclusive. Two groups have reported tha t heavy meromyosin, l ight meromyosin, actin, and t ropomyosin are synthesized and degraded at different rates in vivo (5, 6). Others, however, have suggested tha t the myofibril has a definite life-span (7, 8). We have used the ra t d i a p h r a g m main ta ined in vitro and measured the ra te of incorporat ion of labeled acids into actin, t ropomyosin, an d the heavy and l ight chains of myosin. T h e present studies indicate nonuni form rates of synthesis of the contracti le proteins, and thus suggest heterogeneous rates of degradat ion.