C-Terminal mutations in the chloroplast ATP synthase gamma subunit impair ATP synthesis and stimulate ATP hydrolysis

Interaction between PVY HC-Pro and the NtCF1β-subunit reduces the amount of chloroplast ATP synthase in virus-infected tobacco

The photosynthetic rate of virus-infected plants is always reduced. However, the molecular mechanism underlying this phenomenon remains unclear. The helper component-proteinase (HC-Pro) of Potato virus Y (PVY) was found in the chloroplasts of PVY-infected tobacco, indicating some new function of HC-Pro in the chloroplasts. We generated HC-Pro transgenic plants with a transit peptide to target the protein to chloroplast. The HC-Pro transgenic tobacco showed a decreased photosynthetic rate by 25% at the light intensity of 600 μmol m(-2) s(-1). Using a yeast two-hybrid screening assay to search for chloroplast proteins interacting with HC-Pro, we identified that PVY HC-Pro can interact with the chloroplast ATP synthase NtCF1β-subunit. This interaction was confirmed by GST pull-down and co-immunoprecipitation assays. HC-Pro didn't interfere with the activity of assembled ATP synthase in vitro. The HC-Pro/NtCF1β-subunit interaction might affect the assembly of ATP synthase complex. Quantitative western blot and immunogold labeling of the ATP synthase indicated that the amount of ATP synthase complex was decreased in both the HC-Pro transgenic and the PVY-infected tobacco. These results demonstrate that HC-Pro plays an important role in reducing the photosynthetic rate of PVY-infected plants, which is a completely new role of HC-Pro besides its multiple known functions.

Measurement of chloroplast ATP synthesis activity in Arabidopsis

There are numerous options for monitoring ATP synthesis in chloroplasts using isolated thylakoid membranes, intact chloroplasts, and even whole leaves. Currently, the most commonly used method employs isolated thylakoids coupling the synthesis of ATP to light emission from luciferin in a reaction catalyzed by luciferase. The luciferin-luciferase assay can be highly sensitive and is a direct measure of ATP. Another direct measurement of ATP is the incorporation of 32P into ATP, which, while more technically difficult, has the advantage over the luciferin-luciferase assay of being able to distinguish newly synthesized from total ATP. The phosphorylation of ADP results in a net decrease in pKa (acid disassociation constant) between the reactants and the product ATP, resulting in an increase in the pH of the assay media, which can be used as a convenient, continuous measurement of ATP synthesis. The formation of ΔμH+ across the thylakoid membrane and its concomitant dissipation as ATP is synthesized can be measured by an electrochromic absorption band shift (ECS) of thylakoid pigments measured at 518 nm (Witt, Biochim. Biophys. Acta 505:355-427, 1979; Petty and Jackson, Biochim. Biophys. Acta: Bioenergetics 547:463-473, 1979). The first-order decay time of the ESC can be used to estimate the rate of ATP synthesis providing a noninvasive, indirect method for measuring ATP synthase activity that can be used with intact leaves.

Interaction of pyrophosphate with catalytic and noncatalytic sites of chloroplast ATP synthase

The effect of pyrophosphate (PP(i)) on labeled nucleotide incorporation into noncatalytic sites of chloroplast ATP synthase was studied. In illuminated thylakoid membranes, PP(i) competed with nucleotides for binding to noncatalytic sites. In the dark, PP(i) was capable of tight binding to noncatalytic sites previously vacated by endogenous nucleotides, thereby preventing their subsequent interaction with ADP and ATP. The effect of PP(i) on ATP hydrolysis kinetics was also elucidated. In the dark at micromolar ATP concentrations, PP(i) inhibited ATPase activity of ATP synthase. Addition of PP(i) to the reaction mixture at the step of preliminary illumination inhibited high initial activity of the enzyme, but stimulated its activity during prolonged incubation. These results indicate that the stimulating effect of PP(i) light preincubation with thylakoid membranes on ATPase activity is caused by its binding to ATP synthase noncatalytic sites. The inhibition of ATP synthase results from competition between PP(i) and ATP for binding to catalytic sites.