CHARACTERIZATION OF AN ALKALINE PH-DEPENDENT PROTON SLIP IN THE ATP SYNTHASE OF LETTUCE THYLAKOIDS

An alkaline-pH-dependent increase in basal electron transport is analyzed to be due to a proton 'slip' through the ATP synthase of isolated lettuce thylakoids. The 'slip' was completely blocked by DCCD, unaffected by tentoxin and partially blocked by the presence of magnesium, ADP and phosphate or magnesium and ATP. All the stated components were required to observe the inhibition, suggesting that both the adenine and the γ-phosphate binding position on the ATP synthase must be occupied for the inhibitory effect. The nucleotides were effective in submicromolar concentrations, suggesting binding to a regulatory (non-catalytic) rather than a substrate (catalytic) site on the ATP synthase. The 'slip' modulated by phosphorylating reagents may serve an important regulatory role under physiological conditions. When phosphate availability limits ATP synthesis in the light, the 'slip' will be operative, allowing electron transport to proceed, and preventing damage due to excessive transthylakoid ΔpH. As soon as phosphate becomes available the 'slip' is avoided and normal coupled electron transport is resumed. © 1990.