EVIDENCE FOR MYOSIN MOTORS ON ORGANELLES IN SQUID AXOPLASM

Squid axoplasm has proved a rich source for the identification of moto rs involved in organelle transport. Recently, squid axoplasmic organel les have been shown to move on invisible tracks that are sensitive to cytochalasin, suggesting that these tracks are actin filaments. Here, an assay is described that permits observation of organelles moving on unipolar actin bundles. This assay is used to demonstrate that axopla smic organelles move on actin filaments in the barbed-end direction, s uggesting the presence of a myosin motor on axoplasmic organelles. Ind eed, axoplasm contains actin-dependent ATPase activity, and a pan-myos in antibody recognized at least four bands in Western blots of axoplas m. An almost-equal-to 235-kDa band copurified in sucrose gradients wit h KI-extracted axoplasmic organelles, and the myosin antibody stained the organelle surfaces by immunogold electron microscopy. The myosin i s present on the surface of at least some axoplasmic organelles and th us may be involved in their transport through the axoplasm, their move ment through the cortical actin in the synapse, or some other aspect o f axonal function.