The architecture of the GroEL-GroES-(ADP)(7) chaperonin complex. II. Heptagrammal characterization of the folding.

Abstract

The heptagrammal forms derived in part I [Janner (2003a). Acta Cryst. D59, 783-794] enclose chain segments of symmetry-related monomers in the GroEL-GroES-(ADP)(7) chaperonin complex. A chain reaching the boundary of a given form either ends, proceeds to a neighbouring form or has to fold. C(alpha) atoms corresponding to these folding points are identified in each of the nine forms of the chaperonin and are approximated by ideal positions having integral coordinates (the indices) with respect to a symmetry-adapted basis. Mutual structural relations between the indexed positions are derived in terms of integral scale-rotations (similar to those that leave the form invariant). The magnesium ions at the binding sites of the nucleotides ADP and ATP are shown to be symmetry-related to these folding points. The change in folding (polymorphism) observed in the cis ring of GroEL arising from binding to GroES is discussed. In particular, the form segmentation is conserved in the polymorphic transition. The geometric and algebraic restrictions imposed on the indexed positions and on their structural relations by the integrality condition are presented in an appendix.