Aspects of subunit interactions in the chloroplast ATP synthase. II. Characterization of a chloroplast coupling factor 1-subunit III complex from spinach thylakoids
1993
Wetzel, C.M. | McCarty, R.E.
A complex between chloroplast-coupling factor 1 (CF(1)) and subunit III of the membrane-spanning portion of the chloroplast ATP synthase (CF(0)), isolated as described in the accompanying paper (C.M. Wetzel and R.E. McCarty [1993] Plant Physiol 102: 241-249), has been further characterized. A comparison of the ATPase activities of CF(1), CF(1)-subunit III, and the chloroplast ATP synthase (CF(1)-CF(0)) holoenzyme revealed that the properties of CF(1)-subunit III more closely resemble those of CF(1)-CF(0) than those of CF(1). In particular, the Ca2+-ATPase activity after reduction of the enzyme with dithiothreitol was much lower in CF(1)-subunit III and CF(1)-CF(0) than in CF(1), suggesting that the association of the inhibitory epsilon subunit is tightened by the presence of either CF(0) or subunit III. Cold stability is a property of CF(1)-CF(0) in thylakoid membranes. The ATPase activity of CF(1) incubated in the cold in the presence of asolectin liposomes was lost more rapidly than that of either CF(1)-subunit III or CF(1)-CF(0) incorporated into liposomes. Removal of the epsilon subunit from all three preparations resulted in marked stimulation of their ATPase activity. Although subunit III was also removed during depletion of the epsilon subunit, it is not known whether the two subunits interact directly. CF(1) deficient in the epsilon subunit binds to liposomes containing either subunit III or CF(0). Taken together, these results provide evidence that the association of CF(1) and subunit III of CF(0) is specific and may play a role in enzyme regulation.
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