Specialized conservation of surface loops of myosin: Evidence that loops are involved in determining functional characteristics

The molecular motor myosin has been the focus of considerable structure-fun ction analysis. Of key interest are the portions of the protein that contro l the rate of ATP hydrolysis, the affinity for actin, and the velocity at w hich myosin moves actin. Two regions that have been implicated in determini ng these parameters are the "loop" regions at the junctions of the 25 kDa a nd 50 kDa domains and the 50 kDa and 20 kDa domains of the protein. However , the sequences of these regions are poorly conserved between different myo sin families, suggesting that they are not constrained evolutionarily, and thus are relatively unimportant for myosin function. In order to address th is apparent incongruity, we have performed an analysis of relative rates of observed evolutionary change. We found that the sequences of these loop re gions appear to be actually more constrained than the sequences of the rest of the myosin molecule, when myosins are compared that are known to be kin etically or developmentally similar. This suggests that these loop regions could play an important role in myosin function and supports the idea that they are involved in modulating the specific kinetic characteristics that f unctionally differentiate one myosin isoform from another. Apparently "unco nserved" loops may generally play a role in determining kinetic properties of enzymes, and similar analyses of relative rates of evolution may prove u seful for the study of structure-function relationships in other protein fa milies. (C) 1999 Academic Press.