The largest protein observed to fold by two-state kinetic mechanism does not obey contact-order correlation.

To probe the folding-energy landscape for a very large protein, we used Borrelia burgdorferi VlsE as a model. VlsE is a single-domain, predominantly alpha-helical protein with 341 residues. Remarkably, time-resolved folding and unfolding processes for VlsE follow two-state behavior. VlsE is by far the largest protein characterized that folds by a two-state kinetic mechanism. Thus, the common rule of thumb, that proteins larger than 110 residues fold by complex, multistate kinetic mechanisms, must be used with caution. In contrast with smaller helical proteins, the folding speed in water for VlsE is slow (5 +/- 2 s-1, pH 7, 20 degrees C) and does not agree (by 4 orders of magnitude in different directions) with the speeds predicted on the basis of native-state contact order and the topomer-search model. It is therefore questionable if the barrier height for folding is defined by gross topology for large two-state folders.