Crystal structure of pea Toc34 - a novel GTPase of the chloroplast protein translocon

Chwan-Deng Hsiao,^a Yuh-Ju Sun,^a,b Farhad Forouhar,^a and Hsou-min Li^a

^aInstitute of Molecular Biology, Academia Sinica, Taipei, Taiwan 115, ROC; ^bInstitute of Bioinformatics and Structural Biology, National Tsing Hua University, Hsinchu, Taiwan 300, ROC (mbhsiao@ccvax.sinica.edu.tw)

Toc34, a 34-kDa integral membrane protein, is a member of the Toc (translocon at the outer-envelope membrane of chloroplasts) complex that associates with precursor proteins during protein transport across the chloroplast outer membrane [1-3]. Here we report the crystal structure of the cytosolic part of pea Toc34 complexed with GDP and Mg2+ at 2.0 Å resolution [4]. In the crystal, the Toc34 molecules exist as dimers with features resembling the ones found in a small GTPase complexed with a GTPase activating protein (GAP). Gel-filtration, however, revealed that dimeric and monomeric forms of Toc34 coexisted in phosphate saline buffer (pH 7.2) solution. Mutation of Arg 128, an essential residue for dimerization, to alanine led to the formation of only a monomeric form whose GTPase activity is significantly reduced compared to that of the wild-type Toc34. These results together with a number of structural features unique to Toc34, suggest that each monomer acts as a GAP on the other interacting monomer.

References

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4 Sun, Y.J., Forouhar, F., Li, H. M., Tu, S. L., Yeh, Y. H., Kao, S., Shr, H. L., Chou, C. C., Chen, C., and Hsiao, C. D. (2002) Nature Struct. Biol. 9, 95-100.