Domains, motions and regulation in the myosin head

The motions of muscle must ultimately be related to the motions of myosin heads. Intact muscle fibres studied during activity by X-ray diffraction or optical methods reveal rapid large-scale movements of the heads (Cooke et al., 1982; Huxley et al., 1982; Burghardt et al., 1983; see also review by Irving, 1987). The asynchrony of these motions, however, as well as the limited structural resolution of these techniques, has prevented formulation of a precise description of the structural changes in myosin involved in force production. A recent model that accommodates many of the observations invokes inter-domain motions within the myosin head during activity (Huxley & Kress, 1985). Here we review evidence on the domain structure of the myosin head, and infer where and to what extent internal motions are likely to occur. We also suggest that regulation of myosin's activity may be closely linked to control of these motions. We make use of the notion of 'domains' in myosin, and define a domain as an independent folding unit that has a stable structure (Rossman & Argos, 1981; Kim & Baldwin, 1982). Proteolysis often reveals the size, and in favourable cases also the boundaries of t h e s e folding units. In myosin, two types of domain have been distinguished. The head and rod regions are examples of proteolytically defined domains that can be readily separated and appear to retain their primary functions in isolation. Proteolysis has also been taken to define the boundaries of smaller domains within the head (Baiint et al., 1978; Mornet et al., 1981a). The thermal denaturation profiles of certain head fragments confirm their status as independent folding un i t s (cf. Burke et al., 1987). Caution is required, however, in assigning discrete functions to single domains in a globular protein such as the head. In fibrous proteins a linear arrangement of domains may signify such a one-to-one corres,pondence. However, function in globular proteins often depends critically on inter-domain contacts, which bring together regions of the polypeptide chain that are far apart in the amino acid sequence, and may also change the structures of the domains themselves (cf. Janin & Wodak, 1983; Bennett & Huber, 1984). Thus in the myosin head, full activity may depend on the way the domain structures are modulated by interactions with other domains (cf. Castellani et al., 1987). At the gene level, protein domains are sometimes coded by single exons (cf. Blake, 1978; Go, 1981), but this is probably not the case in the myosin head. In 10 genomic DNA sequences that have been analysed, from 0 to 20 introns interrupt the myosin heavy chain gene in the region that codes for the head (Strehler et al., 1986; Warrick et al., 1986). Although a number of intron positions are conserved, there is only one sequence in which there are (non-conserved) intron positions that correspond to the boundaries of the proteolytic domains (Strehler et al., 1986). Morphologica] evidence gives information on the shape and disposition of the domains, and suggests that the myosin head contains a globular distal region and an extended 'neck' (Fig. 1). Electron microscope images of negatively stained molecules display clefts that divide the head into two large or one large and two smaller regions (Walker et al., 1985). Threedimensional image reconstruction of heads bound to actin (Taylor & Amos, 1981; Vibert & Craig, 1982; Toyoshima & Wakabayashi, 1985; Milligan & Flicker, 1987) also reveals the presence of two or more morphological units; these appear to correspond to the regions in the head defined by image processing of electron micrographs of thin sections of $1 crystals (Winkelmann et al., 1985; Baker & Winkelmann, 1986). Single particle averaging of shadowed myosin heads supports the view that a cleft separates the neck from the globular region (Vibert, 1988). The simplest interpretation of these results is that the