SECRETION BY CONTRACTILE PROTEINS 



73 



concentration, indicating that there is a limited supply of the 

 transporting molecules in the cell membrane. 



If we consider Goldacre's proposal in the light of Osterhout's 

 work and of the studies on phosphatase, it appears that we can 

 meet the requirements of the situation by a model such as that in- 



Adsorption_ 

 centre 



-*~e 



U 



d 



+-U 



Plasma membrane 



-Contractile protein 



^•Phosphatase or phosphokinase 

 enzymic centre 



A.T.P. or other 

 phosphate ester 



^~ Enzymic centre 

 Adsorption centre 



0or 

 0OPO 3 H 2 



Fig. 4. Diagram to illustrate one of the possible mechanisms for transport- 

 ing the substance across a membrane as the result of the action of a 



contractile enzyme. 



dicated in Fig. 4. This model is based upon the view that the 

 secretory activity is performed by the folding of a contractile 

 protein, and that the contraction is brought about by reaction of 

 the protein with a substance such as A.T.P. (adenosine triphos- 

 phate), as is the case with actomyosin. The phosphatase thus 

 represents an enzymic centre through which the energy of A.T.P. 

 is transferred to the contractile protein. In this model, as indi- 

 cated on the upper part of the figure, the initial configuration 

 of the protein has one part of an extended chain on the cyto- 



