FUNCTIONS OF CONTRACTILE PROTEINS 75 



transport such diverse molecules as sugar, potassium, amino 

 acids, etc. It should also be quite possible for a modified form 

 of this mechanism to secure removal of a substance from a cell, 

 instead of secretion of a substance into a cell. 



It is clear that investigation of the actual existence of the 

 mechanism proposed above would be very much assisted if spe- 

 cific poisons were available for the alkaline phosphatases and 

 phosphokinases, particularly those which may react with phos- 

 phorylating agents such as adenosine triphosphate. 



Generalisation of the Contractile Protein 



Hypothesis 



In this chapter alkaline phosphatase has been shown to be 

 present in a variety of cellular sites, and to be fulfilling a variety 

 of functions. It has been my endeavour to indicate that many 

 alternative explanations must be considered in each case. But, 

 to conclude this chapter, I wish to propose a unifying theory, 

 i.e., a theory which would permit alkaline phosphatase to be con- 

 cerned in a wide variety of activities, and yet to be acting in 

 fundamentally the same way in all cases. This theory is that the 

 phosphatase usually represents the enzymic centre of a contrac- 

 tile protein. The fundamental significance of the enzyme activity 

 is the ability to make the chemical energy of a high-energy phos- 

 phate bond available for contraction of a protein. 



In the preceding section of this book we have seen that this 

 view, in conjunction with Goldacre's theory of the performance 

 of osmotic work, affords a most interesting approach to the prob- 

 lems of secretion. We shall now consider the significance of 

 phosphatase in relation to three other problems: (1) chromo- 

 somes, chromocentres and nucleoli; (2) collagen; (3) division 

 spindles. 



1. One of the most striking features of the behaviour of chro- 

 mosomes is their contraction during mitosis and meiosis. The 

 fact that chromosomes commonly have alkaline phosphatase, and 

 that during mitosis they appear to be rich in phosphatase, ap- 

 pears to be compatible with the view that the contraction may be 

 brought about by a process fundamentally similar to that respon- 

 sible for the contraction of actomyosin. In the intermitotic 



