PARENT SESSION

Symposium S1A Proton-coupled electon transport and ATPase
Monday August 30th, 2004 10:20 AM-12:20 PM Room 511D
Chair: Colin Wraight
Co-Chair: Wolfgang Junge

A novel behavior of -subunit of chloroplast ATP synthase. Kaori Kohzuma^*,1, Kinya Akashi¹, Toru Hisabori^{2, 3}, Hanayo Ueoka-Nakanishi³, Naoyuki Inagaki Inagaki¹, Akiho Yokota¹, ¹ Grad. Sch. Biol. Sci., Nara Inst. Sci. Tech. (NAIST), Ikoma, Nara, Japan² Chem. Resources Lab. Tokyo Inst. Tech., Yokohama, Japan³ ATP System Project, ERATO, JST, Yokohama, Japan

ABSTRACT- Chloroplast ATP synthase is composed of the water-soluble complex facing the stroma (CF₁) and the membrane-embeded, hydrophorbic complex (CF_o) and synthesize ATP by utilizing a proton gradient formed across thylakoid membranes. Biochemical functions of individual subunits of these complexes have been examined extensively and well characterized. The -subunit binds to the -subunit and suppresses ATPase activity of CF₁ and the leakage of protons through CF_o. However, there has been little information as to how CF₁/CF_o is regulated depending on physiological conditions, as has been observed with photosystem II. Excess light and drought are major environmental stresses to plants. Under these stresses, the carbon fixation stops and neither ATP nor NADPH is consumed. Wild watermelon plants from the Kalahari Desert survive excess light under drought conditions and are expected to have a mechanism(s) to cope with these conditions. We have been analyzing the change in the protein composition of thylakoid membranes of their leaves during the progress of high light/drought stress by 2D-electrophoresis. We found that the amount of a peptide of 15 kDa decreased to one-tenth of the original level during the stress. The peptide was digested with trypsin and subjected to ESI Q-TOF MS. The peptide was the -subunit of CF₁. Western blotting of the extract of the leaves stressed for different days revealed that the amount of-subunit in the leaves decreased gradually with time without any change in those of - and -subunits. These results suggest that only the -subunit is released to the stroma and digested during the stress. The release of the -subunit from CF₁ under the stress may be involved in the relaxation of hyper-energization of thylakoid membranes.

KEY WORDS: -subunit , ATP synthase , excess light