Exploring the Folding Mechanism of Small Proteins GB1 and LB1.

The computational atomistic description of the folding reactions of the B1 domains, GB1 and LB1, of protein G and protein L, respectively, is an important challenge in current protein folding studies. Although the two proteins have overall very similar backbone structures (β-hairpin-α-helix-β-hairpin), their apparent folding behaviors observed experimentally were remarkably different. LB1 folds in a two-state manner with the single-exponential kinetics, whereas GB1 folds in a more complex manner with an early stage intermediate that may exist on the folding pathway. Here, we used a new method of all-atom molecular dynamics simulations to investigate the folding mechanisms of GB1 and LB1. With the Lorentzian energy term derived from the native structure, we successfully observed frequent folding and unfolding events in the simulations at a high temperature (414 K for GB1 or 393 K for LB1) for both the proteins. Three and two transition-state structures were predicted for the GB1 and LB1 folding, respectively, at the high temperature. Two of the three transition-state structures of GB1 have a better formed second β-hairpin. One of the LB1 transition states has a better formed first hairpin, and the other has both hairpins equally formed. The structural features of these transition states are in good agreement with experimental transition-state analysis. At 300 K, more complex folding processes were observed in the simulations for both the proteins. Several intermediate structures were predicted for the two proteins, which led to the conclusion that both the proteins folded through similar mechanisms. However, the intermediate state accumulated in a sufficient amount only in the GB1 folding, which led to the double-exponential feature of its folding kinetics. On the other hand, the LB1 folding kinetics were well fitted by a single-exponential function. These results are fully consistent with those previously observed experimentally.