The periplasmic molecular chaperone protein SurA binds a peptide motif that is characteristic of integral outer membrane proteins

The Escherichia coli SurA protein is a periplasmic molecular chaperone that facilitates correct folding of outer membrane porins. The peptide binding specificity of SurA has been characterized using phage display of heptameric peptides of random sequence. The consensus binding pattern of aromatic-polar-aromatic-nonpolarproline amino acids emerges for both SurA and a SurA "core domain," which remains after deletion of a peripheral peptidyl-proline isomerase domain. Isothermal titration calorimetry with a high affinity heptameric peptide of sequence WEYIPNV yields peptide affinities in the range of 1 - 14 muM for both SurA and its core domain. Although the peptide consensus aromatic-polar-aromatic- nonpolar-proline occurs infrequently in E. coli proteins, the less restrictive tripeptide motif aromatic-random- aromatic appears with greater-than-random frequency in outer membrane proteins and is prevalent in the " aromatic bands" of the porin beta barrel structures. Thus, SurA recognizes a peptide motif that is characteristic of integral outer membrane proteins.