Buried hydrophobic side‐chains essential for the folding of the parallel β‐helix domains of the P22 tailspike

The processive β‐strands and turns of a polypeptide parallel β‐helix represent one of the topologically simplest β‐sheet folds. The three subunits of the tailspike adhesin of phage P22 each contain 13 rungs of a parallel β‐helix followed by an interdigitated section of triple‐stranded β‐helix. Long stacks of hydrophobic residues dominate the elongated buried core of these two β‐helix domains and extend into the core of the contiguous triple β‐prism domain. To test whether these side‐chain stacks represent essential residues for driving the chain into the correct fold, each of three stacked phenylalanine residues within the buried core were substituted with less bulky amino acids. The mutant chains with alanine in place of phenylalanine were defective in intracellular folding. The chains accumulated exclusively in the aggregated inclusion body state regardless of temperature of folding. These severe folding defects indicate that the stacked phenylalanine residues are essential for correct parallel β‐helix folding. Replacement of the same phenylalanine residues with valine or leucine also impaired folding in vivo, but with less severity. Mutants were also constructed in a second buried stack that extends into the intertwined triple‐stranded β‐helix and contiguous β‐prism regions of the protein. These mutants exhibited severe defects in later stages of chain folding or assembly, accumulating as misfolded but soluble multimeric species. The results indicate that the formation of the buried hydrophobic stacks is critical for the correct folding of the parallel β‐helix, triple‐stranded β‐helix, and β‐prism domains in the tailspike protein.