BIOMOLECULAR ASPECTS OF SPERMATOZOAN MOTILITY 215 



conclusions of Hayashi et ah, 1958, on the role of actin in actomyosin 

 contraction), but it is difficult to study the mechanical properties of 

 actin alone in the way that myosin can be studied since actin is soluble 

 and does not form precipitates or threads except under unfavorable 

 conditions (e.g., in acetone/ water mixtures). Experiments with ace- 

 tone precipitates lightly fixed with formaldehyde suggested that al- 

 though such crosslinked actin gels were not reactive in the sense that 

 actomyosin is reactive, the insoluble specimens were nevertheless 

 capable of swelling and deswelling to a much greater degree than 

 myosin. 



During an examination of films of F-actin dried down from solu- 

 tions containing traces of salts, it was found that although most of 

 these films were insoluble, the outer rims of each plaque were trans- 

 parent, fibrous in appearance, birefringent, and generally oriented 

 tangentially. Fragments of these outer portions swelled in ion-free 

 water and with ATP. Lowering the pH to 4.0 resulted in further 

 swelling and eventual solution. Addition of neutral KC1 produced 

 much the same effect. Diminution of the pH in the presence of KC1, 

 however, resulted in a spectacular syneresis of the specimen, which 

 shrank rapidly, becoming a dense gel rather like syneresed acto- 

 myosin. Three preparations of this kind before and after addition 

 of KC1 at pH 3-4 are shown in Fig. 12. In the first example (Fig. 12a 

 and b) the actin film has shrunk after 1 min, but as in the case of 

 actomyosin and flagellar protein treated with ATP, etc. (see Figs. 

 8b and 9b for comparison), some parts of the gel have remained inert. 

 In the second example (Fig. 12c and <-/), intense syneresis has taken 

 place in an actin film in 3 min, and in contracting to the dense pellet 

 shown in Fig. \2d, a debris of fine particles has been ejected from 

 the body of the specimen. In the third example (Fig. 12<? and /) the 

 gel specimen of actin has contracted vigorously, but once again some 

 parts of the gel appear to be unreactive. In those birefringent films 

 that suggested a certain amount of orientation, the swollen gels 

 shrank along the axis of orientation, often with a folding or buckling 

 of the film. In some instances the gels contracted to about 30% of 

 their original length. On the other hand, specimens prepared from 

 the center of the dried plaques or from old actin specimens were 

 inert, and the behavior of the more reactive preparations suggests 

 that these particular gels may represent a form of F-actin with suf- 



