Probing enzyme quaternary structure by combinatorial mutagenesis and selection.

Genetic selection provides an effective way to obtain active catalysts from a diverse population of protein variants. We have used this tool to investigate the role of loop sequences in determining the quaternary structure of a domain-swapped enzyme. By inserting random loops of four to seven residues into a dimeric chorismate mutase and selecting for functional variants by genetic complementation, we have obtained and characterized both monomeric and hexameric enzymes that retain considerable catalytic activity. The low percentage of active proteins recovered from these selection experiments indicates that relatively few loop sequences permit a change in quaternary structure without affecting active site structure. The results of our experiments suggest further that protein stability can be an important driving force in the evolution of oligomeric proteins.