  The authors address the problem of downhill protein folding in the framework
of a simple statistical mechanical model, which allows an exact solution for
the equilibrium and a semianalytical treatment of the kinetics. Focusing on
protein 1BBL, a candidate for downhill folding behavior, and comparing it to
the WW domain of protein PIN1, a two-state folder of comparable size, the
authors show that there are qualitative differences in both the equilibrium and
kinetic properties of the two molecules. However, the barrierless scenario
which would be expected if 1BBL were a true downhill folder is observed only at
low enough temperature.
