Structural Studies on the Conformations of Myosin

Abstract

Myosin is the major motor protein found in vertebrate striated and smooth muscle and in non-muscle cells where, in association with actin, its main role is to convert chemical energy into mechanical work. Smooth muscle and non-muscle myosin adopts a number of different conformations: for example an unfolded (6S) form which is capable of forming filaments and generating force and a ‘folded’ (10S) form, which is most probably a storage form incapable of forming filaments. In the 10S form the products of ATP cleavage are trapped by the folded tails and the ATPase activity of myosin is greatly reduced. It is believed that a transition to the unfolded 6S form is necessary prior to filament formation. We report here on two relatively low resolution structural techniques for studying hydrated myosin. We have used a relatively recent development in microscopy, the scanning tunneling microscope, to image a series of biologically interesting specimen, mainly to evaluate the potential of the technique. There are significant potential advantages for imaging biological specimen with the STM as the imaging is done in air and the specimen can be imaged without a metal coating. Our experience with imaging myosin suggests that good images of hydrated myosin can be obtained but with poor reproducibilty. We have also carried out small angle solution x-ray scattering studies on the two myosin conformations to explore the possibilities of doing kinetic measurements on the transition between the two states. Small angle scattering from the S1 fragment and re-constituted parts of the rod have also been carried out and the data is compared with expected scattering from model structures.