Genetic techniques for enhancing biochemical and structural characterization of Dictyostelium myosin II

Amino acids 519-524 of Dictyostelium myosin II form a surface loop that aids actin binding by facilitating a conformational change

Residues 519-524 of Dictyostelium myosin II form a small surface loop on the actin binding face, and have been suggested to bind directly to actin through high affinity hydrophobic interactions. To test this hypothesis, we have characterized mutant myosins that lack this loop in vivo and in vitro. A mutant myosin in which this loop was replaced by an Ala residue (delta519-524/+A) was non-functional in vivo. Replacement with a single Gly residue instead of Ala yielded partial function, suggesting that structural flexibility, rather than hydrophobicity, is the key feature of the loop. The in vivo phenotype of the mutant enabled us to identify a number of additional amino acid changes that restore function to the delta519-524/+A mutation. Intriguingly, many of these, including L596S, were located at some distances away from the 519-524 loop. We have also isolated suppressors for the L596S mutant myosin, which was not functional in vivo. The suppressors for delta519-524/+A and those for L596S showed complementary charge patterns. In ATPase assays, delta519-524/+A S1 showed very low activity and little enhancement by actin, whereas L596S S1 was hyper active and displayed enhanced affinity for actin. In motility assays, delta519-524/+A myosin released actin filaments upon addition of ATP and was unable to support movements. L596S myosin was also inactive, but in this case actin filaments stayed immobile even after the addition of ATP. Transient kinetic measurements demonstrated that delta519-524/+A S1 is not only slower than wild type to bind actin filaments, but also slower to dissociate from actin filaments. Based on these results, we concluded that the 519-524 loop is not a major actin binding site but aids actin binding by facilitating a critical conformational change.

Requirement of domain-domain interaction for conformational change and functional ATP hydrolysis in myosin

Coordination between the nucleotide-binding site and the converter domain of myosin is essential for its ATP-dependent motor activities. To unveil the communication pathway between these two sites, we investigated contact between side chains of Phe-482 in the relay helix and Gly-680 in the SH1-SH2 helix. F482A myosin, in which Phe-482 was changed to alanine with a smaller side chain, was not functional in vivo. In vitro, F482A myosin did not move actin filaments and the Mg2+-ATPase activity of F482A myosin was hardly activated by actin. Phosphate burst and tryptophan fluorescence analyses, as well as fluorescence resonance energy transfer measurements to estimate the movements of the lever arm domain, indicated that the transition from the open state to the closed state, which precedes ATP hydrolysis, is very slow. In contrast, F482A/G680F doubly mutated myosin was functional in vivo and in vitro. The fact that a larger side chain at the 680th position suppresses the defects of F482A myosin suggests that the defects are caused by insufficient contact between side chains of Ala-482 and Gly-680. Thus, the contact between these two side chains appears to play an important role in the coordinated conformational changes and subsequent ATP hydrolysis.