Context-dependence of amino acid residue pairing in antiparallel beta-sheets

In an effort to understand the driving forces behind antiparallel beta-sheet assembly, we have investigated the mutational tolerance of four pairs of residues in CspA, the major cold shock protein of E. coli. Two buried pairs and two exposed pairs of neighboring amino acids were separately randomized and the corresponding effects on protein stability were assessed using a protein expression screen. The thermal denaturation of a subset of the recovered proteins was measured by circular dichroism spectroscopy in order to determine the range of stabilities sampled by the expressed mutants. As anticipated, buried sites are substantially less tolerant of substitutions than exposed sites with more than half of the exposed residue combinations giving rise to stably folded proteins. The two exposed residue pairs, however, display different degrees of tolerance to substitution and accept different residue pair combinations. Except for the prohibition of proline from interior strand positions, no obvious correlations of mutant stability with any single parameter such as beta-sheet propensity or hydrophobicity can be detected. Mutant combinations recovered in both orientations (e.g. XY and YX) at a given exposed pair site often show markedly different stabilities, indicating that the local environment plays a substantial role in modulating the pairing preferences of residues in beta-sheets.