Kinesin

Kinesin is the name given to a class of motor proteins found in biological cells.

Function
In the cell, small molecules such as gases and glucose diffuse to where they are needed. Large molecules synthesised in the cell body, intracellular components such as vesicles, and organelles such as mitochondria are too large (and the cytosol too crowded) to diffuse to their destinations. Most kinesins transport such cargo about the cell by walking unidirectionally along microtubule tracks hydrolysing one molecule of adenosine triphosphate (ATP) at each step. It was thought that ATP hydrolysis powered the kinesin walk but it now seems that the force of binding to the microtubule is what pulls the cargo along while the binding of ATP assists the direction of motion.

Structure
The typical kinesin is a protein dimer consisting of two heavy chains and two light chains. The heavy chains comprise a globular head (the motor domain) connected via a short, flexible neck linker to the stalk - a long, central coiled-coil region - that ends in a tail region formed with a light-chain. The stalks intertwine to form the kinesin dimer. Cargo binds to the tail while the twin heads alternately bind the microtubule as the kinesin pulls the cargo along. The heads will hydrolyze 2 ATP molecules per step.

Polarity
Motor proteins travel in a specific direction along a microtubule. This is because the microtubule is polar, the heads only bind to the microtubule in one orientation, and ATP hydrolysis drives the molecule in one direction.

Most kinesins walk towards the positive end of a microtubule which, in most cells, entails transporting cargo from the centre of the cell towards the periphery. This form of transport is known as anterograde transport.

Some kinesins {EG5}, and a different type of motor protein known as dyneins, move towards the minus end of the microtubule. Thus they transport cargo from the periphery of the cell towards the centre. This is known as retrograde transport. These motors have a different morphology: their structure is such that they move in the opposite direction though the directional principle is the same as for the rest of the family.

Proposed mechanisms
Kinesin accomplishes transport by essentially "walking" along a microtubule. Two mechanisms have been proposed to explain how this movement occurs. Despite some remaining controversy, mounting academic evidence points towards the symmetric inchworm mechanism as being more likely.
 * In the "hand-over-hand" mechanism, the kinesin heads step over one another, alternating the lead position.
 * In the "inchworm" mechanism, one kinesin head always leads, moving forward a step before the trailing head catches up.

Asters and assembly
In recent years, it has been found that microtubule-based molecular motors (including a number of kinesins) have a role in mitosis (cell division). The mechanism by which the cytoskeleton of the daughter cell separates from that of the mother cell was unclear. It seems that motors organize the two separate microtubule asters into a metastable structure independent of any external positional cues. This self-organization is in turn dependent on the directionality of the motors as well as their processivity (ability to walk). Thus motors are necessary for the formation of the mitotic spindle assemblies that perform chromosome separation. Specifically, proteins from the Kinesin 13 family act as regulators of microtubule dynamics. The prototypical member of this family is MCAK (formerly Kif2C, XKCM1, Gene ) which acts at the ends of microtubule polymers to depolymerize them. The function of MCAK in cells and its mechanism in vitro is currently being investigated by numerous labs.