Proteins that switch folds
Monday, December 5, 2PM – 3PM
John Orban, University of Maryland
How protein sequence codes for 3D structure remains a fundamental mystery in biology. One approach to understanding the folding code is to design a pair of proteins with maximal sequence identity but retaining different folds. The non-identities must therefore be responsible for determining which fold topology prevails and constitute a “fold-specific folding code”. We used two wild-type domains of streptococcal protein G, the human serum albumin binding domain (GA) and the IgG-binding domain (GB), as our starting points for designing high sequence identity proteins with different folds. GA is a 3-helix bundle while GB has a 4beta+alpha structure. Using a combination of phage display and site-directed mutagenesis, we were able to co-evolve these proteins to 98% sequence identity while maintaining their distinct wild-type folds. Biophysical characterization of structure in solution was determined using NMR spectroscopy. Thus, alternative fold space can be accessed with just a single amino acid mutation in a small protein.
The classic examples of conformational switching in nature, such as prions, involve multimerization as a quaternary driving force for fold change. Here, however, the folds are monomeric and maintain their native functions, albeit with reduced affinity, up until the switch point. Thus multimerization is not a pre-requisite for fold switching. Our results are consistent with accepted principles of protein folding and suggest that large-scale conformational switching through relatively short mutational pathways may be a more facile process for evolving new folds and functions than previously thought.
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