226 FREDERICK G. E. PAUTARD 



ellipsoid with a low (8%) helix value (Kay, 1960) possibly due to the 

 high proline content (Kominz et al., 1954), and although there is dis- 

 agreement about the state of aggregation of the G-form (Kay, 1960), 

 the conditions of polymerization into the F-form have been well es- 

 tablished. Contraction of a string of molecular beads of this kind will 

 depend on the behavior of the polypeptide chains coiled within each 

 bead with respect to the fiber axis, and if the network formed with 

 this kind of fiber has weak side-to-side linkages, then the contraction 

 of the network is the sum of all the filaments sliding over each other. 

 If the side-to-side linkages are too weak, the protein will go into solu- 

 tion. If the filaments are strongly crosslinked, the network will be 

 constrained and inert. Syneresis may presuppose only linear linkages, 

 and in muscle and flagella, this may be the special property of the 

 globular component — where typical globular behavior is accurately 

 synchronized by the arrangement of the end-to-end linkages. In serum 

 albumin, Bresler (1958) found an increase in the axial ratio from 4 to 

 16 at pH 10 when the solvent was made more hydrophobic, and simi- 

 lar intraglobular transformations in a linear polymer would lead to 

 marked changes in length (Astbury, 1958). 



Bresler (1958) also found no changes in optical rotation during 

 changes in shape of serum albumin. This suggests that the helices 

 took no part in the process. Changes of pH apparently have little 

 effect on helices too, so that in actin, the more random and freely 

 movable portions of the molecule may be responsible for the changes 

 in length, leaving the more stable helical regions to move among 

 themselves. There is no indication of the way in which the cross-/? 

 configuration is generated by this system, and further experiments 

 are needed before speculation on the molecular arrangement is possi- 

 ble. It seems clear, however, that these kinds of globular molecules 

 have the dual function of a supporting skeleton with movable arms 

 and legs. 



Actin does seem to conform to the kind of structure that has been 

 postulated by many authors as an electrostatic and osmotic engine, 

 but there is no evidence to suggest how the principles that have been 

 derived from considerations of polyelectrolytes (Katchalsky, 1951, for 

 instance) fit into the behavior of the actin or actomyosin networks. 

 Suffice to say, the changes in actin (and flagellar gels) during contrac- 

 tion and extension have some features apparently unconnected with 



