ABSTRACT
About 25 years ago, forces of ~0.2 pN per myosin head were recorded between myo-sin or myosin heads (S1) and F-actin filaments in vitro, at ~25°C (Kishino and Yanagida 1988). In these experiments, a siliconed glass surface was coated with isolated myosin heads or whole myosin molecules (two heads per molecule) and F-actin filaments were moved over this surface. The authors suggested that the forces recorded during this ‘sliding’ should be multiplied by a factor of ~2-3, ‘because the orientation of myosin and S-1 on the surface are random and therefore all heads are not expected to be able to interact with the actin’, giving forces of ~0.2 × (2-3) ~ 0.4-0.6 pN per head. Kishino and Yanagida (1988) considered these values of ~0.4-0.6 pN per head to be ‘comparable to force exerted in muscle during isometric contraction (about 1 pN)’. The estimate of ~1 pN per head was proposed more than 35 years ago by Oosawa (1977). Woledge (1988) criticised all these values and demonstrated that the isometric force in muscle is higher than ~3 pN per crossbridge (presumably at ~0°C), corresponding to ~4.7-13.0 pN per cross-bridge at ~25°C (maximal interval; see below for the two possible values of Q10). As a cross-bridge contains 1 or 2 heads and there is a wide margin of uncertainty, the values of ~4.7-13.0 pN also correspond to 1 head. The values of ~0.4-0.6 pN per head cannot therefore be considered good estimates for the value in muscle. Some months after his publication with Kishino, and after Woledge’s demonstration
(1988), Yanagida claimed publicly that there was a calibration error in the experiments of Kishino and Yanagida (1988) and that their recorded forces should be multiplied by an additional factor of 2, giving forces of ~(0.4-0.6) × 2 ~ 0.8-1.2 pN per head at ~25°C. Averaging the four corrected values obtained at ~25°C by Kishino and Yanagida (1988) and suggested by Yanagida, we obtain the most probable mean value of ~0.8 pN per head. Taking Q10 ~ 1.20, as for the isometric tetanic tension recorded in intact frog fibres, between ~0°C and ~40°C-45°C (see p. 245 in Section 8.8), the mean force of ~0.8 pN per head at ~25°C corresponds to ~0.6 pN per head at 10°C (the reference temperature used in this monograph). Taking Q10 ~ 1.8 between ~25°C and 10°C, valid for demembranated fibres from rabbit psoas muscle (see p. 246 in Section 8.8), the mean force of ~0.8 pN at ~25°C becomes ~0.3 pN per head at 10°C. Many experiments were performed after those reported by Kishino and Yanagida (1988). For instance, Finer et al. (1994), working at 21°C, found that the force per HMM (two heads S1 + S2; see Figure 5.1 for definition of S1 and S2) ‘covered a broad distribution, ranging from 1 pN to 7 pN’. The values of 1 pN to 7 pN, at 21°C, give values ranging between ~0.5 pN and ~5.6 pN per HMM at 10°C (see above for the two values of the Q10; no correction performed to take into account the existence of two heads in HMM; see the next citation). Molloy et al. (1995a,b) found values of ~1.7-1.8 pN at 23°C, that is, a maximal interval of ~0.8 pN to ~1.4 pN at 10°C, and
they stated that ‘HMM and S1 produced approximately the same amount of force’ (this conclusion is similar to that drawn in the next paragraph, concerning the discussion of the experimental results obtained for S1 and myosin by Kishino and Yanagida 1988, i.e. the impossibility of distinguishing between the forces generated by S1 and by a whole myosin molecule). The ‘stepping forces’ obtained, at room temperature (~22°C), on myosin V (a two-headed myosin) involved in the transport of vesicles organelles and proteins within cells, were of the order of magnitude of ~1-2 pN per head (e.g. Cappello 2008; Cappello et al. 2007; Dunn and Spudich 2007; Mehta 2001; Sellers and Veigel 2006; Yildiz et al. 2003), that is, ~0.5 pN or ~1.6 pN per head at 10°C (maximal interval). Averaging the eight available values, valid at 10°C, we obtain a mean estimate of ~1.4 pN per head. This value is significantly lower than the most probable force of ~7.4 pN per head at 10°C, in an intact unit cell (see p. 88 in Section 3.6). Most specialists in in vitro motility consider the strength of the forces they record in vitro to be comparable to that estimated in an intact fibre, providing support for swinging crossbridge/lever-arm processes. However, neither A.F. Huxley (2000) nor Morel and D’hahan (2000) fully endorse this interpretation (see below, in this chapter). The views of A.F. Huxley (2000) and Morel and D’hahan (2000) are supported by the two different ‘optimal’ values (at 10°C) of ~1.4 pN per head, in vitro, and ~7.4 pN per head, in vivo (see above).
