The Kinesin Molecular Motor
Certain eukaryotic cells face problems moving materials from one place to another in the cytoplasm. Most cells use the endomembrane system as an intracellular highway, the Golgi apparatus packaging materials into vesicles that move through the channels of the endoplasmic reticulum to the far reaches of the cell. However, this highway is only effective over short distances. When a cell has to transport materials through long extensions like the axon of a nerve cell, then the highways of the ER are too slow. For these situations, eukaryotic cells have developed highspeed locomotives that run along tracks of microtubules.

Embedded within the membrane of long-distance transport vesicles are linking proteins that bind the vesicle specifically to one of two types of motor proteins. Nature's tiniest motors, these motor proteins literally pull the transport vesicles along the microtubule tracks. One such vesicle linking protein, kinectin, is an integral membrane protein of the endoplasmic reticulum. Kinectin binds vesicles displaying it on their membranes to the motor protein, kinesin, which uses ATP to power its movement towards the cell periphery, dragging the vesicle with it as it travels along the microtubule. Another vesicle protein (or perhaps a slight modification of kinesin--further research will be needed to say which) binds vesicles to the motor protein dynein, which directs movement in the opposite direction. The destination reached by a particular transport vesicle and its contents is thus determined by the nature of the linking protein embedded within the vesicle's membrane.

Source: "Hitching a Ride" Science, March 24, 1995