Rerouting the Folding Pathway of the Notch Ankyrin Domain by Reshaping the Energy Landscape

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Abstract
The modular nature of repeat proteins has made them a successful target for protein design. Ankyrin repeat, TPR, and leucine rich repeat domains that have been designed solely on consensus information have been shown to have higher thermostability than their biological counterparts. We have previously shown that we can reshape the energy landscape of a repeat protein by adding multiple C-terminal consensus ankyrin repeats to the five N-terminal repeats of the Notch ankyrin domain. Here we explore how the folding mechanism responds to reshaping of the energy landscape. We have used analogous substitutions of a conserved alanine with glycine in each repeat to determine the distribution of structure in the transition state ensembles of constructs containing one (Nank1–5C1) and two consensus (Nank1–5C2) ankyrin repeats. Whereas folding of the wild-type Notch ankyrin domain is slowed by substitutions in its central repeats,1 folding of Nank1–5C1 and Nank1–5C2 is slowed by substitutions in the C-terminal repeats. Thus, the addition of C-terminal stabilizing repeats shifts the transition state ensemble toward the C-terminal repeats, rerouting the folding pathway of the ankyrin repeat domain. These findings indicate that, for the Notch ankyrin domain, folding pathways are selected based on local energetics.