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Note the movement of the red "nose" which responds to ATP binding. The position of the nose probably controls weak or strong actin binding.
This was arrived at by fitting the actin and myosin coordinates into the 18A cryofiltered em reconstruction of decorated actin. These results are to be published soon by Schroeder, Jahn and Holmes.
The s1 fragment has been cut into two pieces: the body, comprising the greater part of the s1 fragment; and the nose, a clump of alpha-helices which comprise the greater part of the 50k upper domain. These are connected with the rest of the molecule by loops 1 and 2 and a loop near residue 400 which is disordered in smooth muscle myosin (Dominguez et al). Two orientations of the nose are shown: nose_initial, which come from fitting the s1 fragment as a solid body to the em map and nose, which is given by a least squares fitting of the body and nose independently (each with 6 degrees of freedom).
It is suggested that nose_initial to nose represents a substantial part of the conformational change from weak to strong binding.