Sequence swapping does not result in conformation swapping for the β4/β5 and β8/β9 β-hairpin turns in human acidic fibroblast growth factor

Abstract

The β-turn is the most common type of nonrepetitive structure in globular proteins, comprising ∼25% of all residues; however, a detailed understanding of effects of specific residues upon β-turn stability and conformation is lacking. Human acidic fibroblast growth factor (FGF-1) is a member of the β-trefoil superfold and contains a total of five β-hairpin structures (antiparallel β-sheets connected by a reverse turn). β-Turns related by the characteristic threefold structural symmetry of this superfold exhibit different primary structures, and in some cases, different secondary structures. As such, they represent a useful system with which to study the role that turn sequences play in determining structure, stability, and folding of the protein. Two turns related by the threefold structural symmetry, the β4/β5 and β8/β9 turns, were subjected to both sequence-swapping and poly-glycine substitution mutations, and the effects upon stability, folding, and structure were investigated. In the wild-type protein these turns are of identical length, but exhibit different conformations. These conformations were observed to be retained during sequence-swapping and glycine substitution mutagenesis. The results indicate that the β-turn structure at these positions is not determined by the turn sequence. Structural analysis suggests that residues flanking the turn are a primary structural determinant of the conformation within the turn.