Motor proteins

General introduction to motor proteins

ATP-driven motor proteins use chemical energy gained by splitting ATP into ADP and Pi at various locations in the cell to generate mechanical movement and force for diverse purposes; motility (myosin, kinesin, dynein), contraction (myosin), channeling (FoF1 ATPase) as well as folding (GroEL), unfolding (Clp, proteasome) and disassembling (NSF) protein substrates. Although these ATP-driven motor proteins share the core architecture around ATP-binding site (called Walker-A motif) and mechanism of ATP hydrolysis, final mechanical movement diverges quite widely, depending on their localizations and roles in the cell. For example, 1) characteristics of movement vary from translational (myosin, kinesin, dynein) and rotational (FoF1 ATPase) motion to folding/unfolding/disassembling of the substrates; 2) various partners of movement such as cytoskeletal proteins (actin for myosin, microtubule for kinesin and dynein), Fo channel for F1 ATPase, peptidase for Clp protease, and many regulators for dynein; 3) movements vary in terms of velocity and orientation – even those ATP-driven motors that belong to the same myosin superfamily move at various speeds and in opposite directions. One of the most interesting questions about the mechanism of these molecules is how the small (2-3 Å) common structural changes at the ATP-binding core are converted into the variety of movements with large (~10-100 Å per one ATP hydrolysis) displacements.

Recent progress in light microscopy and single molecule manipulation made it possible to observe single events in a chemical reaction like ATP hydrolysis, manipulate single molecule and measure forces generated by single molecule. On the other hand, a number of methodologies of structural biology (X-ray and electron crystallography, NMR and cryo-electron microscopy) have been developed to visualize the structure of motor proteins. However, all of these methods only provide us with information about the averaged structure of studied molecules. In order to understand the mechanism of motion of these molecules, it is necessary to investigate structures of individual molecules adopting discrete conformational states.

On the other hand, the recent progress of single molecule manipulation with various kinds of light microscopy enabled the measurement of dynamic motion of individual molecules, measurement of the force generated by single molecule, and the direct visualization of single chemical events such as ATP-hydrolysis. Consequently, it is pointed out that thermal fluctuation plays an important role (“Brownian ratchets”) at the scale of motor proteins (5~20nm, 10~50pN).

However, in order to connect these methodologies (dynamic movement detected by light microscopy, high-resolution information from structural biology, and the behavior of proteins determined in vivo and in vitro), new approaches are necessary. Especially, visualization of individual molecules at high resolution is required for structural understanding of motion and thermal fluctuation.