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Degradation of myofibrillar proteins by μ‐ and m‐ calpain

The initial report characterizing the effects of m‐calpain on the myofibrillar proteins [Dayton et al., Cold Spring Harbor Conferences on Cell Proliferation, Vol.2, pp. 551–577 (1975)] indicated that m‐calpain rapidly degraded troponin T and more slowly degraded troponin I, tropomyosin, and C‐protein. No degradation of actin, α‐actinin, or of myosin heavy or light chains could be detected suggesting a unique and very restricted subsite specificity for m‐calpain. Subsequent studies have confirmed that both μ‐ and m‐calpain have a selective subsite specificity, but several reports have suggested that m‐calpain degrades both myosin [Pemrick and Greenau, J. Cell Biol. 99, 2297 (1984)] and actin [Nagainis et al. J. Food Biochem. 7, 247 (1983)]. We have used μ‐ and m‐calpain purified from either bovine skeletal muscle or human placenta to determine whether the calpains degrade myosin or actin. As measured by either SDS‐PAGE or Western analysis, skeletal muscle actin is not degraded after incubation with either μ‐ or m–calpain for up to 60 min in the presence of 3 mM Ca2+. Both calpains, however, do degrade skeletal muscle myosin slowly under these conditions. In myofibrils, the LC1 myosin light chain is degraded to a single polypeptide of ~ 19‐kDa but this fragment is not degraded any further. No degradation of LC1 is observed when purified myosin is incubated with calpains, and no degradation of LC2 or LC3 light chains is detectable in purified myosin or myofibrils after 60 min of incubation. The myosin heavy chain is degraded to several polypeptides of ~150‐ and 170‐kDa; these fragments do not seem to be degraded further during longer incubation.