A number of viruses exhibit microtubule-based bidirectional transport following cell entry. This behavior raises three questions: First, what mediates their transport along microtubules? Second, how do viruses recruit the motor proteins? Finally, how do they go to the right place by bidirectional transport in a variety of cell types with different microtubule organizations? We studied these questions with Adenovirus 5 (Ad5), a virus with well characterized, dynein-mediated minus transport mechanism. One form of plus end directed motor, Kif5C, has been reported to disrupt Ad5 capsids at the Nuclear Pore Complexes(NPC), but the mechanisms and roles of microtubule plus end-directed Ad5 transport prior to this stage are largely unknown. Here we performed a RNAi screen of 38 microtuble plus end-directed kinesins, which implicated Kif5B (kinesin-1 family) in plus-end directed Ad5 transport, along with several other forms of kinesin. Kif5B knockdown caused an accumulation of Ad5 particles near the centrosomes in human pulmonary epithelial A549 cells.

This effect was strongly enhanced by blocking Ad5 nuclear pore targeting with Leptomycin B and supports a role for Kif5B in Ad5 transport prior to NPC docking. Kif5B RNAi was rescued by expression of any of the three Kif5 orthologues. We also found that Ad5 directly interacts with kinesin-1 via the capsid subunit Penton Base in a PH-independent manner. Together with our earlier studies, these findings reveal that Ad5 has evolved distinct recruitment mechanisms for cytoplasmic dynein and at least one form of kinesin-1 during early infection. Despite clear evidence for short-range linear microtubule-associated Ad5 transport, we found the overall behavior of most Ad5 particles to be stochastic at a larger time scale, by mean-square-displacement (MSD) analysis. We named this behavior "assisted diffusion''. In consistent with this mechanism, Ad5 was able to maintain a normal nuclear targeting after we displaced centrosomes away from the nucleus by inhibiting CDK1 in late G2 cells. We also directly observed Ad5 switching from microtubule based transport to nuclear targeting from a microtubule near the nucleus.

Kif5B RNAi dramatically inhibited this novel microtubule-based random-walk/“assisted-diffusion” mechanism. By super resolution microscopy, we found a more local distribution of NPC attached Ad5 over the entire nuclear surface under conditions of Kif5B knock down. We propose that adenovirus uses independently-recruited kinesin and dynein to fully explore the cytoplasm to search for and dock at the nucleus, a mechanism of potential importance for physiological cargoes as well.