Early endocytic vesicles loaded with Texas Red asialoorosomucoid were prepared from mouse liver. These vesicles bound to microtubules in vitro, and upon ATP addition, they moved bidirectionally, frequently undergoing fission into two daughter vesicles. There was no effect of vanadate (inhibitor of dynein) on motility, whereas 5 -adenylylimido-diphosphate (kinesin inhibitor) was highly inhibitory. Studies with specific antibodies confirmed that dynein was not associated with these vesicles and that Kif5B and the minus-end kinesin Kifc1 mediated their plus-and minus-end motility, respectively. More than 90% of vesicles associated with Kifc1 also contained Kif5B, and inhibition of Kifc1 with antibody resulted in enhancement of plus-end-directed motility. There was reduced vesicle fission when either Kifc1 or Kif5B activity was inhibited by antibody, indicating that the opposing forces resulting from activity of both motors are required for fission to occur. Immunoprecipitation of native Kif5B by FLAG antibody after expression of FLAG-Kifc1 in 293T cells indicates that these two motors can interact with each other. Whether they interact directly or through a complex of potential regulatory proteins will need to be clarified in future studies. However, the present study shows that coordinated activity of these kinesins is essential for motility and processing of early endocytic vesicles.
INTRODUCTIONReceptor-mediated endocytosis is a process in which ligands bind to specific cell surface receptors and internalize via clathrin-coated pits. After internalization the clathrin coat is released and uncoated vesicles mature into early endosomes (Mellman, 1996;Mukherjee et al., 1997;Marsh and McMahon, 1999;Higgins and McMahon, 2002;Perrais and Merrifield, 2005). After acidification of early endosomes, ligands such as asialoorosomucoid (ASOR) that are destined for lysosomes dissociate from their receptors (Harford et al., 1983a,b), and a series of fission events results in their segregation into separate daughter vesicles (Wolkoff et al., 1984;Mellman, 1996;Mukherjee et al., 1997;Murray and Wolkoff, 2003). The resulting ligand-enriched late endosomes traffic to lysosomes for degradation, whereas the receptor-containing daughter endosomes traffic back to the cell surface where receptor is reused (Harford et al., 1983a;Wolkoff et al., 1984;Mukherjee et al., 1997). Previous studies indicated that this segregation event requires an intact microtubule cytoskeleton (Goltz et al., 1992;Novikoff et al., 1996;Murray et al., 2000;Bananis et al., 2003Bananis et al., , 2004. In more recent studies, microtubule-based endosome motility and segregation were reconstituted in vitro using fluorescent early endosomes prepared from rat liver 5 min after portal venous injection of Texas Red-labeled ASOR, a substrate for the hepatocytespecific asialoglycoprotein receptor (ASGPR) (Bananis et al., , 2003(Bananis et al., , 2004Murray et al., 2000;Murray and Wolkoff, 2003). On addition of ATP to the in vitro assay, these vesicles moved bidirectionall...