ACTIN-DEPENDENT ORGANELLE MOVEMENT IN SQUID AXOPLASM

STUDIES of organelle movement in axoplasm extruded from the squid giant axon have led to the basic discoveries of microtubule-dependent organelle motility 1-3 and the characterization of the microtubule-based motor proteins kinesin and cytoplasmic dynein 4,5. Rapid organelle movement in higher animal cells, especially in neurons, is considered to be microtubule-based. The role of actin filaments, which are also abundant in axonal cytoplasm 6,7, has remained unclear. The inhibition of organelle movement in axoplasm by actin-binding proteins 8-11 such as DNase I, gelsolin and synapsin I has been attributed to their ability to disorganize the microtubule domains where most of the actin-filaments are located 7. Here we provide evidence of a new type of organelle movement in squid axoplasm which is independent of both microtubules and microtubule-based motors. This movement is ATP-dependent, unidirectional, actin-dependent, and probably generated by a myosin-like motor. These results demonstrate that an actomyosin-like mechanism can be directly involved in the generation of rapid organelle transport in nerve cells.