Sarcomeric myosin heavy chain is degraded by the proteasome

Cardiac myofibrillar proteins, like all other intracellular proteins, are i n a dynamic state of continual degradation and resynthesis. The balance bet ween these opposing metabolic processes ultimately determines the number of functional contractile units within each cardiac muscle cell. Although alt erations in myofibrillar protein degradation have been shown to contribute to cardiac growth and remodeling, the intracellular proteolytic systems res ponsible for degrading myofibrillar proteins to their constitutive amino ac ids are currently unknown. Lactacystin, a recently developed, highly specif ic proteasome inhibitor, was used in this study to examine the role of the proteasome in myosin heavy chain (MHC) degradation in cultured neonatal rat ventricular myocytes. Cells were treated with growth medium alone or with lactacystin (1-50 mu M) for up to 48 h. Lactacystin significantly increased the total protein/DNA ratio and markedly prolonged MHC half-life. Other pr oteasome inhibitors, namely carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (10 m u M) and N-acetyl-L-leucyl-L-leucyl-norleucinal (100 mu M), were also effec tive in suppressing MHC degradation. Lactacystin and other proteasome inhib itors also suppressed the markedly accelerated MHC degradation associated w ith Ca2+ channel blockade but did not prevent the disassembly and loss of m yofibrils that accompanied contractile arrest. Thus, sarcomere disassembly precedes the degradation of MHC, which is at least in part mediated by the proteasome.