Temperature plasticity of contractile proteins in fish muscle

Abstract

SUMMARY Three myosin heavy chain isoforms with different actin-activated Mg2+-ATPase activities were found in the fast skeletal muscle from carp (Cyprinus carpio) acclimated to 10 and 30°C. The composition of three types of myosin heavy chain was dependent on acclimation temperature,demonstrating the presence of temperature-specific myosin isoforms in carp. Subsequently, the temperature-dependence of the sliding velocity of fluorescent F-actin in myosins isolated from 10°C- and 30°C-acclimated carp was measured. At 8°C, the filament velocity was three times higher for myosin from 10°C- than from 30°C-acclimated fish. Activation energies (Ea) for the sliding velocity of F-actin were 63 and 111 kJ mol-1 for myosins from 10°C- and 30°C-acclimated fish, respectively. Activation energy for actin-activated Mg2+-ATPase activity was 0.46 kJ mol-1 in myosin from 10°C-acclimated fish and 0.54 kJ mol-1 in myosin from 30°C-acclimated fish. The inactivation rate constant(KD) of Ca2+-ATPase was 7.5×10-4s-1 at 30°C for myosin from 10°C-acclimated fish, which was approximately twice that for myosin from 30°C-acclimated fish. It is suggested that these differences in thermostability reflect a more flexible structure of the myosin molecule in cold-acclimated carp, which results in a reduced activation enthalpy for contraction and, hence, a higher sliding velocity at low temperatures. Structural analysis of cDNAs encoding the carp myosin heavy chain demonstrated striking differences in two surface loops of myosin subfragment-1 (S1), loops 1 and 2, between the 10°C and 30°C types, which were predominantly expressed in carp acclimated to 10°C and 30°C, respectively. Chimeric myosins composed of Dictyostelium discoideum myosin backbones with loop sequences of carp S1 heavy chain isoforms demonstrated that the diversity of the loop 2 sequence of carp S1 affected the Vmax of actin-activated Mg2+-ATPase activity.