Structure And Functions Of Kinesin's ?4 Helix

Neck-linker docking to the motor domain is the force-generation process of kinesin.The driving force of neck-linker docking originates from the binding of ATP molecule to nucleotide-binding pocket of kinesin.Neck linker and nucleotide-binding pocket locates on the opposite side of kinesin's motor domain.The motor-domain rotation which mainly occurs on the core ?-sheet bridges the nucleotide-binding pocket and neck linker and plays both force-transmitting and amplification roles.?4 helix is the longest helix structure of kinesin's motor domain.It is also an important microtubule-binding site of kinesin.The ?4helix is an amphipathic helix.The charged and polar residues mainly locate in the interface between ?4 and microtubule and the hydrophobic residues mainly locate in the interface between ?4 and motor domain.Both sides of ?4 match perfectly with motor domain and microtubule and play very important role in the motor-domain rotation.Using molecular dynamics simulation,we quantitatively analysis the interactions between ?4 helix and microtubule/motor domain at different nucleotide-binding states.We find that the lower side of ?4 has interactional and geometrical match with microtubule through hydrogen bonds and salt bridges to lean firmly against microtubule and provide the fulcrum for motor-domain rotation.In the motor-domain rotation process,?4 helix is fixed on the microtubule and form the axis of this rotation.The upper side of ?4 consists of hydrophobic residues of motor domain(especially the core ?-sheet).This hydrophobic contact forms the hydrophobic interface between ?4 and motor domain and facilitates the motor-domain rotation process.The polar and charge interactions of ?4's lower side with microtubule and the hydrophobic contact between ?4's upper side with core ?-sheet make ?4 function as a lubricated shaft and ensure the reduction of energy consumption required for kinesin's walking process.We also find that the hydrogen bond(Ser257-His205)provides the important constraining force for motor-domain rotation.