COrWROL OF CHLOROPLAST STRUCTURE BY ADENOSINE TRIPHOSPHATE 



Lester Packer 



It is now clear that the membranes of mitochondria (l-2), 

 chloroplasts (3), and photosynthetic bacteria (k) manifest struc- 

 tural changes that are coupled to energy transfer reactions. 

 These changes in membrane structure when measured by the record- 

 ing of a physical parameter such as light- scattering are found to 

 be closely correlated with conditions favorable for either oxida- 

 tive or photophosphorylation. Since the reactants required for 

 phosphorylation coupled to electron flow are also necessary for 

 demonstrating structural changes in both photosynthetic and non- 

 photosynthetic systems, and further, since inhibition of these 

 functions abolish the structural changes, it has been suggested 

 that the structural parameter is under the control of dark energy 

 containing intennediates (l-^). 



Conditions which affect ATP hydrolysis and synthesis have of- 

 ten been implicated in the control of membrane structure. In 

 mitochondria (5-6) and mitochondrial membrane fragments (T-8), 

 ATP plus a divalent cation such as Mg++, may serve to drive struc- 

 tural changes, which, in the case of intact mitochondria involve 

 shrinl^age. Recent experiments with mitochondria (9) have demon- 

 strated that swelling induced by electron transport is oligomycin 

 ijisensifeLve, whereas reversal of swelling by ATP (under conditions 

 of oxidative phosphorylation) is specifically blocked by oligo- 

 mycin. These observations suggest that ATP and its hydrolysis 

 can be involved in the control of raechanochemical changes. Ohni- 

 shi and Ohnishi (lO) have isolated a protein from mitochondria 

 which undergoes conformational changes with ATP (as indicated by 

 light- scattering and viscosity changes) and manifests ATPase acti- 

 vity. It is therefore an attractive hypothesis that in mitochon- 

 dria, a contractile- like substance may be involved in the changes 

 in macromolecular structure coupled to energy transduction. 



The action of ATP on chloroplast structure has only recently 

 been investigated (11-12). Itoh, Izawa and Shibata (12) using 

 the Coulter counter have discovered that whole spinach chloro- 

 plasts shrink reversibly in the light to become 50-80^0 of their 

 previous volume. The time course of this phenomenon was relative- 

 ly slow, requiring one to two hours in the dark for reversal of 



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