Investigating the functional role of a buried interchain aromatic cluster in Escherichia coli GrpE dimer

Aromatic clusters in the core of proteins are often involved in imparting structural stability to proteins. However, their functional importance is not always clear. In this study, we investigate the thermosensing role of a phenylalanine cluster present in the GrpE homodimer. GrpE, which acts as a nucleotide exchange factor for the molecular chaperone DnaK, is well known for its thermosensing activity resulting from temperature-dependent structural changes that allow control of chaperone function. Using mutational analysis, we show that an interchain phenylalanine cluster in a four-helix bundle of the GrpE homodimer assists in the thermosensing ability of the co-chaperone. Substitution of aromatic residues with hydrophobic ones in the core of the four-helix bundle reduces the thermal stability of the bundle and that of a connected coiled-coil domain, which impacts thermosensing. Cell growth assays and SEM images of the mutants show filamentous growth of Escherichia coli cells at 42°C, which corroborates with the defect in thermosensing. Our work suggests that the interchain edge-to-face aromatic cluster is important for the propagation of the structural signal from the coiled-coil domain to the four-helical bundle of GrpE, thus facilitating GrpE-mediated thermosensing in bacteria.