Diagram of proposed myosin-I action in phagocytic cup constriction compared with myosin-II function in the contraction of a cleavage furrow. (a) Schematic cross-section through a cleavage furrow. Contraction of the cleavage furrow in mitotic cells is reinforced by the conventional myosin-II that forms bipolar filaments with the motor domains pointing into opposite directions (blue). In this simplified scheme, actin filaments (red chevron polymers) attached with their (+) ends to the plasma membrane are assumed to point into the cytoplasmic space of the cleavage furrow, as suggested previously . By moving in the (+) end direction (blue arrows), the myosin-II filaments apply force on the perimeter of the cleavage furrow, thus causing it to contract. (b) Schematic view of the orifice at the rim of a phagocytic cup at the neck of a budded yeast particle being engulfed (black circle). Actin filaments are again assumed to point from the plasma membrane into the cytoplasmic space. They act against a barrier of branched and cross-linked actin filaments in the cell cortex. Clusters of membrane-bound myosin-I molecules (blue) move by their motor domains in the (+) end direction (blue arrows) and are proposed to allow actin subunits to enter. The force applied against the barrier is postulated to cause the orifice of the bud to constrict or the neck of a particle to be severed (black arrows).