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Asymmetry of the GroEL-GroES Complex under Physiological Conditions as Revealed by Small-Angle X-Ray Scattering

Tomonao Inobe ^*, Kazunobu Takahashi ^* , Kosuke Maki ^* , Sawako Enoki ^*, Kiyoto Kamagata ^*, Akio Kadooka ^*, Munehito Arai ^* and Kunihiro Kuwajima ^*  

^* Department of Physics, School of Science, University of Tokyo and CREST, Japan Science and Technology Agency, Tokyo 113-0033, Japan; Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Okazaki 444-8787, Japan; and Department of Functional Molecular Science, School of Physical Sciences, Graduate University for Advanced Studies SOKENDAI, Okazaki 444-8787, Japan

Correspondence: Address reprint request to Kunihiro Kuwajima, Tel.: 81-564-59-5230; Fax: 81-564-59-5234; E-mail: kuwajima{at}ims.ac.jp.

Despite the well-known functional importance of GroEL-GroES complex formation during the chaperonin cycle, the stoichiometry of the complex has not been clarified. The complex can occur either as an asymmetric 1:1 GroEL-GroES complex or as a symmetric 1:2 GroEL-GroES complex, although it remains uncertain which type is predominant under physiological conditions. To resolve this question, we studied the structure of the GroEL-GroES complex under physiological conditions by small-angle x-ray scattering, which is a powerful technique to directly observe the structure of the protein complex in solution. We evaluated molecular structural parameters, the radius of gyration and the maximum dimension of the complex, from the x-ray scattering patterns under various nucleotide conditions (3 mM ADP, 3 mM ATPS, and 3 mM ATP in 10 mM MgCl₂ and 100 mM KCl) at three different temperatures (10°C, 25°C, and 37°C). We then compared the experimentally observed scattering patterns with those calculated from the known x-ray crystallographic structures of the GroEL-GroES complex. The results clearly demonstrated that the asymmetric complex must be the major species stably present in solution under physiological conditions. On the other hand, in the presence of ATP (3 mM) and beryllium fluoride (10 mM NaF and 300 µM BeCl₂), we observed the formation of a stable symmetric complex, suggesting the existence of a transiently formed symmetric complex during the chaperonin cycle.