No conventional function for the conventional kinesin?

Abstract

A paper by DeGiorgis et al. (DeGiorgis JA, Petukhova TA, Evans TA, Reese TS. Kinesin-3 is an organelle motor in the squid giant axon. Traffic 2008; DOI: 10.1111/j.1600-0854.2008.00809.x) in this issue of Traffic reports on the identification and function of a second squid kinesin, a kinesin-3 motor. As expected, the newly discovered motor associates with axoplasmic organelles in situ and powers motility along microtubules of vesicles isolated from squid axoplasm. Less expected was the finding that kinesin-3 may be the predominant motor for anterograde organelle movement in the squid axon, which challenges the so far undisputed view that this function is fulfilled by the conventional kinesin, kinesin-1. These novel findings let us wonder what the real function of kinesin-1--the most abundant motor in squid axons--actually is.

Figure 1. Vesicle transport without vesicle motility

A model for anterograde and retrograde transport of cargo vesicles by Kinesin-1-driven gliding of short microtubules over long, stationary microtubules. Vesicles are stably attached to the moving microtubule via non-motor, anchoring proteins (not drawn). For anterograde transport, the vesicle-laden, short microtubule is itself a cargo for Kinesin-1, attached to Kinesin-1's tail (A). For retrograde transport, Kinesin-1 motors are stably attached (via their tail) to long, stationary microtubules, and the vesicle-laden, short microtubule is pushed in the retrograde direction (B). Note that a function-blocking anti-Kinesin-1 antibody would inhibit vesicle transport in both directions. A similar model, where short microtubules are transported anterogradely through the action of cytoplasmic dynein (rather than Kinesin-1), stably attached to a quasi-stationary actin-spectrin meshwork, was previously proposed to explain axonal transport of short microtubules (38); such a model may also account for anterograde transport of vesicle-laden microtubules (C). Arrows show direction of movement of the vesicle-laden microtubules.