220 FREDERICK G. E. PAUTARD 



In muscle, the "sliding" mechanism proposed by Hanson and 

 Huxley (1955) has been developed from morphological considerations 

 of the more advanced myofibrils, and the idea of interposition of 

 rigid elements does receive some support from the x-ray diffraction 

 evidence, which does not suggest any change in the molecular ar- 

 rangement after shortening (Astbury, 1947a, for example). On the 

 other hand, there are numerous models for a folding mechanism, and 

 the subject has a long history. Although Meyer and Mark (1930) 

 claimed to be the first to propose a folding of molecules as a primary 

 event in muscular contraction in their objection (see Meyer and 

 Mark, 1951) to the conclusions of Goldacre and Lorch (1950), the 

 physical reality of a folding protein was first demonstrated by Astbury 

 (Astbury and Street, 1932; Astbury and Woods, 1933), who showed 

 that proteins could undergo reversible changes of dimension as a 

 result of actual changes in the configuration of molecules. Numerous 

 models have been proposed since, based on supercontraction (Ast- 

 bury, 1960), crystalline-amorphous changes (for example Flory, 1956), 

 Donnan osmotic and electrostatic effects (developed by W. Kuhn 

 and reviewed by Kuhn et al., 1960). 



In other biological contractile systems, the structures are so diverse 

 that comparison with muscle is difficult. The concept of rigid ele- 

 ments moving over one another has been developed by Noland 

 (1957) to explain protoplasmic streaming, while a mechanism of paral- 

 lel displacement forces has been put forward by Jahn and Rinaldi 

 (1959) to account for the movement of filaments in Allogromia lati- 

 collaris. In contrast, a folding model has been proposed by Goldacre 

 and Lorch (1950) as the basis of amoeboid movement; also a number 

 of composite sliding-folding systems have been suggested (for ex- 

 ample, Frey-Wyssling, 1948). 



Perhaps the most surprising feature of this wealth of theories on 

 movement throughout nature is that many of them have been de- 

 veloped independently of any knowledge of the nature of the con- 

 tractile substance, and where the substance has been identified as 

 actomyosin, most of the theories are detailed without any reference 

 to the properties of actin or myosin. In spite of the considerable litera- 

 ture on the physiology and kinetics of the myofibril, there is little 

 information about the mechanical and mechanochemical properties 

 of actomyosin, very little about myosin, and none at all about actin. 

 It is almost as if the contractile system were a poor model for itself. 



