Myosin is a protein that binds to actin and has ATPase activity that is activated by actin. Myosin IB may have a role in chemotaxis and aggregation; it could serve to stabilize and even retract cortical structures, such as pseudopods and lamellopods. Involved in the whole cell motility of aggregation-stages cells. Overexpression results in significant decrease in the rate of cellular translocation and fluid-phase pinocytosis and abnormalities in the normal rearrangement of the actin cytoskeleton. Myosin I heavy chain is single-headed. Dimer of a heavy and a light chain. Inability to self-assemble into filaments. Highest concentration just beneath the plasma membrane in the anterior pseudopod at the leading edge of the cell and at sites of cell-cell contact in both stationary and aggregation stage cells. Also found in macropinocytic structures. Rises dramatically during early development. Myosin tail domain binds directly to anionic phospholipid membranes; myosins I could therefore move actin relative to membranes and vice versa. TH.2 and SH3 bind tightly to F-actin; this together with the nucleotide-sensitive site in the head, allows single molecules of myosin I to cross-link actin filaments. Shows defects in pseudopod formation and translocation. Also shows slower speed of locomoting, aggregation-stage cells and significant reduction in initial rate of phagocytosis. MyoB and myoC double mutant exhibits profound defects in growth, endocytosis and rearrangement of F-actin. Expression of the full-length myoB heavy chain in these cells fully rescues the double mutant defects. myoB and myoD double mutant exhibits reduction in the speed of whole cell translocation. myoB, myoC and myoD triple mutant exhibits reduction in the speed of whole cell translocation. Belongs to the TRAFAC class myosin-kinesin ATPase superfamily. Myosin family.