The molecular motors kinesin and dynein drive bidirectional motility along
microtubules (MTs) in most eukaryotic cells1,2. Land plants, however, are a notable
exception, since they contain a large number of kinesins but lack cytoplasmic
dynein, the foremost processive retrograde transporter3,4. It remains unclear how
plants achieve retrograde cargo transport without dynein. Here, we have analyzed
the motility of the six members of minus-end-directed kinesin-14 motors in the
moss Physcomitrella patens and found that none are processive as native dimers.
However, when artificially clustered into as little as dimer of dimers, the type-VI
kinesin-14 (a homologue of Arabidopsis KCBP [kinesin-like calmodulin binding
protein]) exhibited highly processive and fast motility (up to 0.6 μm/s). Multiple
kin14-VI dimers attached to liposomes also induced transport of this membrane
cargo over several microns. Consistent with these results, in vivo observations of
GFP-tagged kin14-VI in moss cells revealed fluorescent punctae that moved
processively towards the minus ends of the cytoplasmic MTs. These data suggest
that clustering of a kinesin-14 motor serves as a dynein-independent mechanism for
retrograde transport in plants.