GroEL–Substrate Interactions Molding the Fold, or Folding the Mold?

(K d in the low micromolar range). As a first step, Chen and Sigler formed cocrystals of the SBP peptide and the isolated apical domain of GroEL (residues 191–336), and Molecular chaperones ensure that proteins achieve and solved this structure at high resolution (2.1 A ˚ , see Figure maintain their proper folds in the appropriate cellular 2a). They have also solved and partially refined a struc-compartments. Among this fascinating class of biologi-ture of the SBP peptide complexed with the intact GroEL cal helpers, chaperonins have emerged as magical fold-tetradecamer. It is exciting that we now have atomic ing machines found in the prokaryotic and eukaryotic resolution images of GroEL binding to an oligopeptide cytosol (GroEL/ES and TriC, respectively) as well as in designed to mimic a substrate. However, as a cautionary chloroplasts (cpn60/cpn10) and mitochondria (hsp60/ note, it is important to keep in mind that a single peptide hsp10). Extensive biochemical and structural work model like SBP can not represent the scope and variety has revealed of protein substrate interactions with GroEL. mechanistic details for the function of the paradigm A Flexible Peptide-Binding Groove E. coli chaperonin, GroEL, which acts with its partner When bound to the GroEL apical domain, the SBP pep-cochaperonin GroES as a two-stroke, ATP-regulated tide forms a ␤ hairpin structure and is nestled in a hy-folding machine (Figure 1). The full GroEL tetradecamer drophobic groove between helices H and I of the apical forms two back-to-back rings, each with seven subunits, domain (Figures 2a and 2b). Significantly, this is the which encapsulate two internal cavities where substrate same site that is occupied by the GroES mobile loop proteins are sequestered. The current view is that fold-regions (Landry et al., 1993) in the structure of the GroEL/ ing is facilitated both because the protein substrate is GroES complex (Xu et al., 1997). It is also the same isolated from other cellular constituents, essentially at site that was observed in an earlier crystal structure of infinite dilution, and because GroEL binding can unfold isolated GroEL apical domain to bind a seven-residue or partially unfold kinetically trapped intermediates and N-terminal epitope tag from an adjacent molecule in the give them a fresh start in folding. However, two of the most intriguing aspects of GroEL function have proved difficult to elucidate: the structural details that explain how a multitude of different sub-strates bind to the chaperonin, and the identity …