A naturally occurring repeat protein with high internal sequence identity defines a new class of TPR-like proteins. Pa42PR Structure

Linear repeat proteins often have high structural similarity and low (~25%) pairwise sequence identities (PSI) among modules. We identified a unique P. anserina (Pa) sequence with tetratricopeptide repeat (TPR) homology, which contains longer (42 residue) repeats (42PRs) with an average PSI >91%. We determined the crystal structure of five tandem Pa 42PRs to 1.6Å, and examined the stability and solution properties of constructs containing three to six Pa 42PRs. Compared to 34-residue TPRs (34PRs), Pa 42PRs have a one-turn extension of each helix, and slightly more oblique helix-helix packing angles. Equilibrium unfolding transitions become more stable and sharper as Pa 42PRs are added, suggesting a higher level of cooperativity in folding compared to consensus 34PRs (c34PRs). These results demonstrate the versatility of the TPR motif to length variation, and provide a basis to understand the effects of helix length on intrinsic/interfacial stability using nearest-neighbor (Ising) statistical thermodynamic models.