SurA is a molecular chaperone that is localized in the periplasm of gram-negative bacteria. It is thought to facilitate correct folding/assembly of outer membrane porins. Although sequence analysis shows that SurA has two peptidyl-prolyl isomerase domains, these are not essential for biological activity; however, the non-prolyl-isomerase domains of the sequence are essential. The structure is bipartite; a core domain has an extended, deep cleft, and one prolyl-isomerase domain is tethered ~30 Angstroms from this core. Peptide studies reveal that SurA recognizes an Aromatic-X-Aromatic sequence motif, which is characteristic of the "aromatic bands" of outer membrane porins. The mechanism by which SurA chaperones protein folding remains unclear.