INHIBITION OF CONTRACTILE VACUOLE FUNCTION IN-VIVO BY ANTIBODIES AGAINST MYOSIN-I

MYOSIN-I is thought to supply the force for movement of cell membranes relative to actin filaments (reviewed in refs 1, 2), but confirmation of this hypothesis has been difficult because of the presence of mult iple isoforms of myosin-I and other unconventional myosins in most cel ls3. We report here the first evidence that a myosin-I isoform is esse ntial for a specific class of intracellular membrane movements in vivo . In Acanthamoeba, the contractile vacuole is an autonomous structure which fuses with the plasma membrane to control the water content of t he cell. Because myosin-IC is the only myosin-I isoform concentrated i n the contractile vacuole complex4,5 and a protein antigenically relat ed to myosin-IC is located on or near the Dictyostelium (slime mould) contractile vacuole6, we thought this organelle might provide the best opportunity to demonstrate a relationship between myosin-I and membra ne motility. Antibodies that inhibit the activity of Acanthamoeba myos in-IC in vitro interfere with expulsion of excess water by the contrac tile vacuole in vivo, leading to overfilling of this organelle and cel l lysis. Myosin-IC may generate the force required to contract the vac uole and may also be involved in transfer of water to the contractile vacuole during refilling.