Kinesin

A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP (thus kinesins are ATPases). The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular cargo, such as in axonal transport. Most kinesins walk towards the plus 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.

Overall structure
Members of the kinesin family vary in shape but the typical kinesin is a protein dimer (molecule pair) consisting of two heavy chains and two light chains. The heavy chain comprises a globular head (the motor domain) connected via a short, flexible neck linker to the stalk–a long, central alpha-helical stalk–that ends in a tail region formed with a light-chain. The stalks intertwine to form the kinesin dimer. Cargo binds to the light chain.

Kinesin motor domain
The head is the signature of kinesin and its amino acid sequence is well conserved among various kinesins. Each head has two separate binding sites: one for the microtubule and the other for ATP. ATP binding and hydrolysis as well as ADP release change the conformation of the microtubule-binding domains and the orientation of the neck linker with respect to the head; this results in the motion of the kinesin. Several structural elements in the head, including a central beta-sheet domain and the Switch I and II domains, have been implicated as mediating the interactions between the two binding sites and the neck domain. Kinesins are related structurally to G proteins, which hydrolyze GTP instead of ATP. Several structural elements are shared between the two families, notably the Switch I and Switch II domains.

Cargo transport
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. Motor proteins fulfill the role of transporting large cargo about the cell to their required destinations. Kinesins are motor proteins that transport such cargo by walking unidirectionally along microtubule tracks hydrolysing one molecule of adenosine triphosphate (ATP) at each step. It was thought that ATP hydrolysis powered each step, the energy released propelling the head forwards to the next binding site. However, it has been proposed that the head diffuses forward and the force of binding to the microtubule is what pulls the cargo along.

There is significant evidence that cargoes in-vivo are transported by multiple motors.

Direction of motion
Motor proteins travel in a specific direction along a microtubule. This is because the microtubule is polar and the heads only bind to the microtubule in one orientation, while ATP binding gives each step its direction through a process known as neck linker zippering.

Most kinesins walk towards the plus 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. Kinesin-14 family proteins, such as Drosophila NCD, budding yeast KAR3, and Arabidopsis ATK5, walk in the opposite direction, toward microtubule minus ends.

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, for example from the terminal buttons of a neuronal axon to the cell body (soma). This is known as retrograde transport.

Proposed mechanisms of movement
Kinesin accomplishes transport by "walking" along a microtubule. Two mechanisms have been proposed to account for this movement. Despite some remaining controversy, mounting experimental evidence points towards the hand-over-hand mechanism as being more likely.
 * In the "hand-over-hand" mechanism, the kinesin heads step past 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.

ATP binding and hydrolysis cause kinesin to travel via a "seesaw mechanism" about a pivot point.

Theoretical Modeling of Kinesin
A number of theoretical models of the molecular motor protein Kinesin have been proposed. Many challenges are encountered in theoretical investigations given the remaining uncertainties about the roles of protein structures, precise way energy from ATP is transformed into mechanical work, and the roles played by thermal fluctuations. This is a rather active area of research. There is a need especially for approaches which better make a link with the molecular architecture of the protein and data obtained from experimental investigations.

Kinesin and mitosis
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.

Family members
Human kinesin family members include following proteins:
 * 1A – KIF1A, 1B – KIF1B
 * 2A – KIF2A, 2C – KIF2C
 * 3B – KIF3B, 3C – KIF3C
 * 4A – KIF4A, 4B – KIF4B
 * 5A – KIF5A, 5B – KIF5B, 5C – KIF5C
 * 6 – KIF6
 * 7 – KIF7
 * 9 – KIF9
 * 11 – KIF11
 * 12 – KIF12
 * 13A – KIF13A, 13B – KIF13B
 * 14 – KIF14
 * 15 – KIF15
 * 16B – KIF16B
 * 17 – KIF17
 * 18A – KIF18A, 18B – KIF18B
 * 19 – KIF19
 * 20A – KIF20A, 20B – KIF20B
 * 21A – KIF21A, 21B – KIF21B
 * 22 – KIF22
 * 23 – KIF23
 * 24 – KIF24
 * 25 – KIF25
 * 26A – KIF26A, 26B – KIF26B
 * 27 – KIF27
 * C1 – KIFC1, C2 – KIFC2, C3 – KIFC3

light chains:
 * 1 – KLC1, 2 – KLC2, 3 – KLC3, 4 – KLC4