Exploring the conformational properties of the sequence space between two proteins with different folds: an experimental study.

We have examined the conformational properties of 27 polypeptides whose sequences are hybrids of two natural protein domains with 8 % sequence identity and different structures. One of the natural sequences (spectrin SH3 domain) was progressively mutated to get closer to the other sequence (protein G B1 domain), with the only constraint of maintaining the residues at the hydrophobic core. Only two of the mutants are folded, each of them having a large sequence identity with one of the two natural proteins. The rest of the mutants display a wide range of structural properties, but they lack a well-defined three-dimensional structure, a result that is not recognized by computational tools commonly used to evaluate the reliability of structural models. Interestingly, some of the mutants exhibit cooperative thermal denaturation curves and a signal in the near-ultraviolet circular dichroism spectra, both typical features of folded proteins. However, they do not have a well-dispersed nuclear magnetic resonance spectrum indicative of a defined tertiary structure. The results obtained here show that both the hydrophobic core residues and the surface residues are important in determining the structure of the proteins, and suggest that the appearance of a completely new fold from an existing one is unlikely to occur by evolution through a route of folded intermediate sequences.