Crystallographic structure of SurA, a molecular chaperone that facilitates folding of outer membrane porins

The SurA protein facilitates correct folding of outer membrane proteins in gram-negative bacteria. The sequence of Escherichia coli SurA presents four segments, two of which are peptidyl-prolyl isomerases (PPlases); the crystal structure reveals an asymmetric dumbbell, in which the amino-terminal, carboxy-terminal, and first PPlase segments of the sequence form a core structural module, and the second PPlase segment is a satellite domain tethered similar to30 Angstrom from this module. The core module, which is implicated in membrane protein folding, has a novel fold that includes an extended crevice. Crystal contacts show that peptides bind within the crevice, suggesting a model for chaperone activity whereby segments of polypeptide may be repetitively sequestered and released during the membrane protein-folding process.