Probing the folding free energy landscape of the Src-SH3 protein domain.

The mechanism and thermodynamics of folding of the Src homology 3 (SH3) protein domain are characterized at an atomic level through molecular dynamics with importance sampling. This methodology enables the construction of the folding free energy landscape of the protein as a function of representative reaction coordinates. We observe that folding proceeds in a downhill manner under native conditions, with early compaction and structure formation in the hydrophobic sheet consisting of the three central beta strands of the protein. This state bears considerable resemblance to the experimentally determined transition state for folding. Folding proceeds further with the formation of the second hydrophobic sheet consisting of the terminal strands and the RT loop. The final stages of folding appear to involve the formation of the hydrophobic core through the expulsion of water molecules bridging the two hydrophobic sheets. This work sheds new light on the complementary roles of sequence and topology in governing the folding mechanism of small proteins and provides further support for the role of water in facilitating the late stages in folding.