Membrane traffic; microtubule motor cycles and kinectin

Although membrane traffic can occur in the absence of microtubutes, the rapid metabolism of many eucaryobc cells requires that vesicular organelles are actively transported Microtubute-dependent transport is particularly robust in actively growing cells and the volume of membrane transported increases dramatically in going from static to growth conditions. Both inward and outward movement are regulated simultaneously implying coordinate activation of both motors or a transport cycle Relatively large complexes of the microtubule motors and associated proteins are responsible for powering the movements (reviewed in Vallée and Sheetz, 1996). Of the many motors identified, kinesin and cytoplasmic dynem are the most abundant A membranous motor receptor, kinectin, was identified on the basis of its interaction with kinesin (Toyoshima et al., 1992). An antibody to kinectin blocks both kinesin binding to organelles and kmesin-dependent motility of those organelles (Kumar et al., 1995) In addition, the same antibody partially inhibits cytoplasmic dynein-dependent motility. We find that loading of VSP4D into chick embryo fibroblasts inhibits aproxmatety 80% of intracellular organelle movements as viewed by video-enhanced DIC microscopy Further, the reassembly of the Golgi after Brefeldin A induced disassembly and movement of the viral G glycoprotein to the cell surface were also impaired. Recent studies show that the small G proteins. rac1. rhoD and rhoA, bind to subdomams of kinectin and will alter motile activity. Thus, kinectn appears to be an important component for the rapid processing of proteins in the secretory pathway. The suggestion that it serves as a receptor lor both kinesin and cytoplasmic dynein is supported by in vitro binding data wherein the two motors compete for binding to vesicles (Vu et al., 1992) and the alpha helical portion of kinectin (Yu et al, 1995} will bind strongly to both kinesin and cytoplasmic dynein To explain these findings and the linkage between the regulation of motility and other steps in vesicle processing, we have suggested that kinectin is a cental component of the control of the microtubule motor cycle (Sheetz and Yu. 1995) in the membrane traffic process, the steps of fission, transport, fusion and processing could be coordinated through a protein such as kinectin which is part of the membrane being transported.