Cooperative unfolding of a single-layer β-sheet protein, CPAP G-box

CPAP is a centriolar protein and its C-terminal domain, G-box or TCP, has a very unique structure that comprises a single-layer β-sheet without hydrophobic core packing. Here we characterized its biophysical properties, including its stability against chemical denaturation. Interestingly, upon urea-induced equilibrium unfolding, the CPAP G-box showed cooperative unfolding behavior that is the hallmark of globular proteins. We analyzed the m-value, a measure of the cooperative transition, from the urea-induced unfolding and found that the estimated m-value from surface burial upon folding is consistent with the experimental value, supporting the two-state unfolding. Next, we constructed deletion mutants of the terminal β-strands and found that the mutants showed reduced stability. The unique structure and characteristics of CPAP G-box provides an interesting opportunity to observe how the core-less flat β-sheet protein can be folded in solution.