PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Quantitative interplay of Psbs and VDE protonation events in the activation of qE. KENJI TAKIZAWA^*,1, JEFFREY CRUZ¹, DAVID KRAMER¹, ¹ Institute of Biological Chemistry, Washington State University, Pullman, Washington, USA

ABSTRACT- During photosynthesis, light driven trans-thylakoid proton motive force (pmf) drives ATP synthesis and regulates photon capture via the qE mechanism. Two pH dependent processes have been linked to qE quenching: 1) protonation of Psbs protein, and 2) de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin by the protonated violaxanthin de-epoxidase. Induction of qE should depend on the pKa values and the cooperativity of protonation (Hill coefficients, n) of Psbs and VDE. Using in vivo spectroscopy and a range of mutant Arabidopsis plants, we show that Psbs and VDE activation have similar pKa values, but their n values are distinct. In vivo, VDE shows a high n value, consistent with in vitro measurements, whereas Psbs has a much lower n value, perhaps 1-2, consistent with its predicted structure. In wild type plants, protonation of VDE appears to exert more control over the activation of qE than does that of Psbs at low pmf, whereas the reverse appears true at high pmf. We propose that the cooperation of the two pH dependent processes tunes the response of qE to pmf to obtain an appropriate balance between photochemical efficiency and photo-protection.

KEY WORDS: Psbs, proton motive force, qE, VDE