BIOMOLECULAR ASPECTS OF SPERMATOZOAN MOTILITY 225 



logical contraction of the myofibril similarly tend to support the idea 

 of changes in the state of the myosin (Mandelkern et al., 1959). On the 

 other hand, the appearance of a very weak cross-^8 configuration in 

 actomyosin following addition of ATP (Pautard, 1958b) or with 

 physiological changes of pH (Pautard, 1959) has recently been attrib- 

 uted to changes in the actin rather than the myosin. Alternative evi- 

 dence of the stability of the a-configuration in certain circumstances 

 comes from experiments with a-keratoses extracted from oxidized 

 wool and hair. Whereas these a-proteins show neither mechanical nor 

 configurational changes with changes in pH, the residual /^-keratoses 

 (which have no apparent structure when extended) are highly con- 

 tractile and show configurational changes under the same conditions 

 (Pautard and Speakman, 1960). 



These experiments suggest that the globular proteins are more 

 readily capable of configurational change than the fibrous a-proteins, 

 and it is tempting to suggest that when a-proteins do supercontract 

 and give a cross-/? diffraction diagram they must first go through a 

 globular phase, so that the common progenitor, so to speak, is some 

 intermediate state where the protein is neither one thing nor the 

 other. This may be the special significance of proteins like actin, 

 where globular units are aggregated into fibrous polymers. Experi- 

 ments with F-actin gels which show that actin by itself is capable of 

 considerable dimensional change at pH values producing the cross-/? 

 configuration suggest that what happens to a-proteins under vigorous 

 conditions is precisely what happens to F-actin under physiological 

 conditions; "supercontraction" is simply applicable, in extenso, from 

 one to the other. In other words, since every k-m-e-f protein may be 

 both fibrous and globular (see Astbury, 1958, for correlation), F-actin 

 will undergo the same configurational and dimensional changes as 

 those observed during the physiological syneresis of actomyosin, 

 whereas myosin will do so only after more drastic treatment. 



In considering how the actin may undergo dimensional change, it 

 must be noted that whereas it is customary to illustrate a macromole- 

 cule as a thin line, without reference to its shape, globular proteins, 

 which are mixtures of regular and random regions coiled together 

 in various ways, are best illustrated at several levels of structure — the 

 straggling polypeptide chains, the more ordered helical portions and 

 the outline of the whole molecule. In G-actin, the molecule is an 



