  Kinesin superfamily protein UNC-104, a member of the kinesin-3 family,
transports synaptic vesicle precursors (SVPs). In this study, the number of
active UNC-104 molecules hauling a single SVP in axons in the worm
Caenorhabditis elegans was counted by applying a newly developed non-invasive
force measurement technique. The distribution of the force acting on a SVP
transported by UNC-104 was spread out over several clusters, implying the
presence of several force-producing units (FPUs). We then compared the number
of FPUs in the wild-type worms with that in arl-8 gene-deletion mutant worms.
ARL-8 is a SVP-bound arf-like small guanosine triphosphatase, and is known to
promote unlocking of the autoinhibition of the motor, which is critical for
avoiding unnecessary consumption of adenosine triphosphate when the motor does
not bind to a SVP. There were fewer FPUs in the arl-8 mutant worms. This
finding indicates that a lack of ARL-8 decreased the number of active UNC-104
motors, which then led to a decrease in the number of motors responsible for
SVP transport.
