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 intr acellular vesicles. Myosin Is have been proposed as key players for membran e trafficking in endocytosis or exocytosis. In the present paper we provide biochemical and immunoelectron microscopic evidence indicating that a pool of myosin I alpha (MMI alpha) is associated with endosomes and lysosomes. We show that the overproduction of MMI alpha or the production of nonfuncti onal truncated MMI alpha affects the distribution of the endocytic compartm ents. We also show that truncated brush border myosin I proteins, myosin Is that share 78% homology with MMI alpha, promote the dissociation of MMIa! 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. MMI alpha might therefore be involved in membrane trafficking occurring between endosomes and lysosomes.