Motor and cargo interactions.

The movements of intracellular cargo along microtubules within cells are often saltatory or of short duration. Further, calculations of the fraction of membrane vesicles that are moving at any period, indicate that active motor complexes are rare. From observations of normal vesicle traffic in cells, there appears to be position-dependent activation of motors and a balance of traffic in the inward and outward directions. In-vitro binding of motors to cargo is observed under many conditions but motility is not. Multi-component complexes appear to be involved in producing active organelle movements by a graded activation system that is highly localized in the cell. The basis of the activation of motility of the organelle motor complexes is still unknown but phosphorylation has been implicated in many systems. In the case of the motor-binding protein, kinectin, it has been linked to active organelle movements powered by conventional kinesin. From the coiled-coil structure of kinectin and the coiled-coil tail of kinesin, it is postulated that a coiled-coil assembly is responsible for the binding interaction. Many other cargoes are transported but the control of transport will be customized for each function, such as axonemal rafts or cytoskeletal complexes. Each function will have to be analyzed separately and motor activity will need to be integrated into the specific aspects of the function.