Action of Adenosine Triphosphate on Chloroplast Structure.

Mitochondria (1, 2), chloroplasts (3), and photosynthetic bacteria (4) manifest mechanochemical changes that are coupled to energy transfer reactions in their membranes. These changes in membrane structure or membrane deformations, conveniently measured by the recording of a physical parameter, such as light scattering, are closely correlated with conditions favorable for oxidative or photophosphorylation. Since the reactants required for phosphorylation coupled to electron transport are also required for the changes in structure in both photosynthetic and nonphotosynthetic systems, and since inhibitors of these functions abolish the structural changes, it has been suggested that the structural parameter is under the control of energy-containing intermediates (l-4). Conditions which affect the hydrolysis and synthesis of adenosine triphosphate have often been implicated in the control of membrane structure. In mitochondria (5-6) and mitochondrial membrane fragments (7-8), ATP plus a divalent cation such as Mg++ may serve to drive structural changes which, in the case of intact mitochondria, involve shrinkage. Recent experiments with mitochondria (9) have shown that swelling induced by electron transport is insensitive to oligomycin, whereas reversal of swelling by ATP (under conditions of oxidative phosphorylation) is specifically blocked by oligomycin. These observations suggest that ATP and its hydrolysis can be involved in the control of mechanochemical changes. Ohnishi and Ohnishi (10) have isolated a protein from mitochondria which undergoes conformational changes with ATP (as indicated by light scattering and viscosity changes) and manifests adenosine triphosphatase activity. It is therefore an attractive hypothesis that in mit.ochondria, a contractile 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 (la), using the Coulter counter, have discovered that whole spinach chloroplasts shrink reversibly in the light to become 50 to 80% of their previous volume. The time course of this phenomenon is relatively slow, requiring 1 to 2 hours in the dark for reversal of the shrinkage. A more rapid and reversible type of structural change in chloroplasts has also been reported by measurement of changes in “rectilinear attenuance,” which is thought to result from changes in the axial ratio of chloroplasts as shown by a correlated morphological study (12). The addition of ATP to chloroplasts incubated either in the light or in the dark brings about a shrinkage which is quite rapid; however, the circumstances giving rise to this effect appear still somewhat uncertain.*