Thermal stability of carp actin in its polymerized form

We investigated the thermal denaturation of carp F-actin by measuring the loss of polymerization ability. The thermal denaturation rates of carp F-actin were at least 10-fold higher than those of chicken F-actin. Binding of tropomyosin thermally stabilized carp F-actin with no appreciable change in activation energy, suggesting the instability was caused by a high frequency of fragmentation of the actin filaments. A comparison of the critical concentration for polymerization suggested that the subunit–subunit interactions of carp F-actin were indeed weaker than those of chicken F-actin. Furthermore, using fluorescence quenching we showed that the nucleotide binding region of carp F-actin was in a more open conformation. Together, our results suggest that the instability of carp F-actin is a function of both the rate of fragmentation and the dissociation of bound nucleotides.