| Summary: | Abstract Macromolecular crowding effects on globular proteins, which usually adopt a single stable fold, have been widely studied. However, little is known about crowding effects on fold-switching proteins, which reversibly switch between distinct folds. Here we study the mutationally driven switch between the folds of GA and GB, the two 56-amino acid binding domains of protein G, using a structure-based dual-basin model. We show that, in the absence of crowders, the fold populations P A and P B can be controlled by the strengths of contacts in the two folds, κ A and κ B. A population balance, P A ≈ P B, is obtained for κ B/κ A = 0.92. The resulting model protein is subject to crowding at different packing fractions, ϕ c. We find that crowding increases the GB population and reduces the GA population, reaching P B/P A ≈ 4 at ϕ c = 0.44. We analyze the ϕ c-dependence of the crowding-induced GA-to-GB switch using scaled particle theory, which provides a qualitative, but not quantitative, fit of our data, suggesting effects beyond a spherical description of the folds. We show that the terminal regions of the protein chain, which are intrinsically disordered only in GA, play a dominant role in the response of the fold switch to crowding effects.
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