A highly stable protein chimera built from fragments of different folds.

Proteins increased in complexity during the course of evolution. Domains as well as subdomain-sized fragments were recruited and adapted to form new proteins and novel folds. This concept can be used in engineering to construct new proteins. We previously reported the combination of fragments from two ancient protein folds, a flavodoxin-like and a (ßα)8-barrel protein. Here we report two further attempts at engineering a chimeric protein from fragments of these folds. While one of the constructs showed a high tendency to aggregate, the other turned out to be a highly stable, well-structured protein. In terms of stability against heat and chemical denaturation this chimera, named NarLHisF, is superior to the earlier presented CheYHisF. This is the second instance of a chimera build from two different protein folds, which demonstrates how easily recombination can lead to the development and diversification of new proteins--a mechanism that most likely occurred frequently in the course of evolution. Based on the results of the failed and the successful chimera, we discuss important considerations for a general design strategy for fold chimeras.