Protein plasticity to the extreme:: changing the topology of a 4-α-helical bundle with a single amino acid substitution

Background: Conventional wisdom has it that two proteins sharing 98.4% sequence identity have nearly identical three-dimensional structures. Here we provide a counter-example to this statement by showing that a single amino acid substitution can change the topology of a homodimeric Ca-helical bundle protein. Results: We have determined the high-resolution crystal structure of a 4-alpha-helical protein with a single alanine to proline mutation in the turn region, and show that this single amino acid substitution leads to a complete reorganisation of the whole molecule. The protein is converted from the canonical left-handed all-antiparallel form, to a right-handed mixed parallel and antiparallel bundle, which to the best of our knowledge and belief represents a novel topological motif for this class of proteins. Conclusions: The results suggest a possible new mechanism for the creation and evolution of topological motifs, show the importance of loop regions in determining the allowable folding pathways, and illustrate the malleability of protein structures.