Journal of Muscle Research and Cell Motility 9, 344-358 (1988) Actin filament organization and myosin head labelling patterns in vertebrate skeletal muscles in the rigor and weak binding states JOHN M. SQUIRE and JEFFREY J. HARFORD Biophysics Section, The Blackett Laboratory, Imperial College, London SW7 2BZ, U.K. Received 17 July 1987 and in revised form 22 February 1988 Summary The structures of vertebrate skeletal muscles (particularly from frog and fish) in the rigor state are analysed in terms of the concept of target areas on actin filaments. Assuming that 100% of the heads are to be attached to actin in rigor, then satisfactory qualitative low-resolution modelling of observed X-ray diffraction data is obtained if the outer ends of these myosin heads can move axially (total range about 200.~) and azimuthally (total range less than 60 ~ from their original lattice sites on the myosin filament surface to attach in defined target areas on the actin filaments. On this basis, each actin target area comprises about four actin monomers along one of the two long-pitched helical strands of the actin filament (about 200 ~) or an azimuthal range of actin binding sites of about 100 ~around the thin filament axis. If myosin heads simply label in a non-specific way the nearest actin monomers to them, as could occur with non-specific transient attachment in a 'weak binding' state, then the predicted X-ray diffraction pattern would comprise layer lines af the same axial spacings (orders of 429 A) as those seen in patterns from resting muscle. It is shown that actin target areas in vertebrate skeletal muscles are probably arranged on an approximate 62 (right-handed) helix of pitch (P) of about 720 A, subunit translation P/6 and near repeat P/2. Troponin position need hot be considered in defining the labelling pattern of cross-bridges on this 62 helix of target areas; the target areas appear to be defined solely by the azimuthal position of the actin binding sites. The distribution of actin filament labelling patterns could be regular in fish muscle which has a 'crystalline' A-band, but will be irregular in higher vertebrate muscles such as frog sartorius muscle. Introduction Rigor is one of the most well-defined static muscle states and it may involve a myosin cross-bridge interaction with actin that is akin to the end of the active cross-bridge stroke during muscular contrac- tion (Lymn & Taylor, 1971). Furthermore, recent studies of invertebrate muscles have started to reveai the patterns of cross-bridge attachment fo actin that occur in rigor muscle, and this is clearly crucial for a proper understanding of the actin-myosin interac- tion. To date, the main advances have been in the study of asynchronous insect flight muscle in rigor. In the case of Lethoeerus flight muscle in rigor it has been thought that cross-bridges only label the thin filaments in particular "actin target areas', spaced axially about 385/~ apart (Reedy, 1968; Squire, 1972) and not in the intervening 'exclusion zones'. Although considerable controversy remains about the nature of the labelling within each target area (Squire, 1972; Reedy & Garrett, 1977; Offer & Elliott, 1978; Squire, 1979, 1981b; Offer et al., 1981), it has now been shown that some of the most important features of the X-ray diffraction pattern from rigor insect muscle are readily explained by the target area concept (Holmes et al., 1980). Thus the existence of 385~-spaced target areas on thin filaments in Lethocerus muscle is widely accepted, eventhough the labelling in each target area remains to be detailed. A major puzzle that remains concerns the origin of the target areas. Squire (1972) suggested that it was the geometry of interaction of myosin cross-bridges with the helical array of actin attachment sites which defined the target areas. Because of the varying azimuth of the actin monomers along the helix, it was thought that at some axial positions cross-bridge attachment to an adjacent actin filament would be sterically favourable, whereas attachment at other axial positions on the rhin filament (i.e. in the exclu- sion zones) would be much less easy, if hOt impossible. However, it so happens that in Lethocerus 0142-4319/88 $03.00 + .12 9 1988 Chapman and Hall Ltd.