Translocation of vesicles from squid axoplasm on flagellar microtubules

Abstract

Directed intracellular particle movement is a fundamental process characteristic of all cells. During fast axonal transport, membranous organelles move at rapid rates, from 1 to 5 µm s⁻¹, in either the orthograde or retrograde direction along the neurone and can traverse distances as long as 1 m (for reviews, see refs 1–3). Recent studies indicate that this extreme example of intracellular motility can occur along single microtubules⁴, but the molecules generating the motile force have not been identified or localized. It is not known whether the force-transducing ‘motor’ is associated with the moving particle or with the microtubule lattice. To distinguish between these hypotheses and to characterize the membrane–cytoskeletal interactions that occur during vesicle translocations, we have developed a reconstituted model for microtubule-based motility. We isolated axoplasmic vesicles from the giant axon of the squid Loligo pealei as described previously⁵. The vesicles (35–475 nm in diameter) were then added to axonemes of Arbacia punctulata spermatozoa that served as a source of microtubules. Axonemes were used because the tubulin subunit lattice of the A-subfibre of a given outer doublet is the same as the subunit lattice of neuronal microtubules along which motility occurs^6,7. Moreover, all the microtubules of a single axoneme show the same structural polarity^8–10, indicating that the axoneme represents an oriented microtubule substrate. Here we demonstrate that vesicle motility is ATP-dependent, that it is not mediated by the flagellar force-transducing molecule dynein and that the direction of movement is not specified by microtubule polarity.