Conformational dynamics of helix S6 from Shaker potassium channel: simulation studies

Prolines in transmembrane (TM) alpha-helices are believed to play an important structural and/or functional role in membrane proteins. At a structural level a proline residue distorts alpha-helical structure due to the loss of at least one stabilizing backbone hydrogen bond, and introduces flexibility in the helix that may result in substantial kink and swivel motions about the effective "hinge." At a functional level, for example in Kv channels, it is believed that proline-induced molecular hinges may have a direct role in gating, i.e., the conformational change linked to opening/closing the channel to movement of ions. In this article we study the conformational dynamics of the S6 TM helix from of the Kv channel Shaker, which possesses the motif PVP--a motif that is conserved in Kv channels. We perform multiple molecular dynamics simulations of single S6 helices in a membrane-mimetic environment in order to effectively map the kink-swivel conformational space of the protein, exploiting the ability of multiple simulations to achieve greater sampling. We show that the presence of proline locally perturbs the helix, disrupting local dihedral angles and producing local twist and unwinding in the region of the hinge--an effect that is relaxed with distance from the PVP motif. We furthermore show that motions about the hinge are highly anisotropic, reflecting a preferred region of kink-swivel conformation space that may have implications for the gating process.