Actin filaments and myosin I alpha cooperate with microtubules for the movement of lysosomes.

An earlier report suggested that actin and myosin I alpha (MMIalpha), a myosin associated with endosomes and lysosomes, were involved in the delivery of internalized molecules to lysosomes. To determine whether actin and MMIalpha were involved in the movement of lysosomes, we analyzed by time-lapse video microscopy the dynamic of lysosomes in living mouse hepatoma cells (BWTG3 cells), producing green fluorescent protein actin or a nonfunctional domain of MMIalpha. In GFP-actin cells, lysosomes displayed a combination of rapid long-range directional movements dependent on microtubules, short random movements, and pauses, sometimes on actin filaments. We showed that the inhibition of the dynamics of actin filaments by cytochalasin D increased pauses of lysosomes on actin structures, while depolymerization of actin filaments using latrunculin A increased the mobility of lysosomes but impaired the directionality of their long-range movements. The production of a nonfunctional domain of MMIalpha impaired the intracellular distribution of lysosomes and the directionality of their long-range movements. Altogether, our observations indicate for the first time that both actin filaments and MMIalpha contribute to the movement of lysosomes in cooperation with microtubules and their associated molecular motors.

Association of myosin I alpha with endosomes and lysosomes in mammalian cells.

Myosin Is, which constitute a ubiquitous monomeric subclass of myosins with actin-based motor properties, are associated with plasma membrane and intracellular vesicles. Myosin Is have been proposed as key players for membrane trafficking in endocytosis or exocytosis. In the present paper we provide biochemical and immunoelectron microscopic evidence indicating that a pool of myosin I alpha (MMIalpha) is associated with endosomes and lysosomes. We show that the overproduction of MMIalpha or the production of nonfunctional truncated MMIalpha affects the distribution of the endocytic compartments. We also show that truncated brush border myosin I proteins, myosin Is that share 78% homology with MMIalpha, promote the dissociation of MMIalpha from vesicular membranes derived from endocytic compartments. The analysis at the ultrastructural level of cells producing these brush border myosin I truncated proteins shows that the delivery of the fluid phase markers from endosomes to lysosomes is impaired. MMIalpha might therefore be involved in membrane trafficking occurring between endosomes and lysosomes.