Assembled ATP Synthase Research Articles

The chloroplast ATP synthase in Chlamydomonas reinhardtii: I. Characterization of its nine constitutive subunits

We have carried out an analysis of the synthesis, cellular accumulation, and membrane binding of the chloroplast-encoded subunits of the ATP synthase ($\alpha$, $\beta$, $\epsilon$, I, III, and IV) in several mutants of $Chlamydomonas reinhardtii$ defective in photophosphorylation. These data gave some insight on the putative genetic lesion in each mutant and allowed some characterization of the assembly and stabilization of the ATP synthase complex in the thylakoid membranes. Four chloroplast mutants are likely to be altered in chloroplast structural genes coding for coupling factor (CF) 1 subunits $\beta$ and $\epsilon$ and for CF$_o$ subunits I and IV. A fifth chloroplast mutant and three nuclear mutants were altered in genes regulating either transcription or translation of chloroplast genes coding for CF1 sub-units $\alpha$ and $\beta$ and CF$_o$ subunits III and IV. Evidence is presented (i) for a control of the rate of synthesis of subunit $\beta$ by subunit $\alpha$ in the absence of ATP synthase assembly and (ii) for an interaction between $\alpha$ and $\beta$ subunits in the stroma of the chloroplast which protects $\alpha$ subunits from proteolytic degradation. The role of several chloroplast-encoded subunits of CF$_o$ and CF$_1$ in the stabilization of partially assembled ATP synthase is discussed. We conclude that in the absence of ATP synthase assembly, CF$_o$ cannot accumulate in the thylakoid membranes, whereas $\alpha$ and $\beta$ subunits, presumably engaged in soluble CF$_1$, can accumulate in the stroma of the chloroplast.