The motor of amoeboid leucocytes

The elements of cytoplasmic movement are: force generation; orientation of the force to provide direction; a control mechanism. The cortical cytoplasm of amoeboid leucocytes is an isotropic gel meshwork of actin filaments crosslinked at intervals by actin-binding protein. Myosin filaments dispersed within this gel bind to the actin filaments and aggregate them in the presence of magnesium and ATP. Gelsolin, a recently discovered protein, non-covalently severs actin filaments in the presence of submicromolar concentrations of ionized calcium, effecting a gel to sol transformation. This reaction reverses when the calcium concentration falls. In a complete system consisting of purified actin, myosin and gelsolin, calcium gradients control the direction of movement of the protein mixture. Since these proteins are in the peripheral cytoplasm of amoeboid leucocytes, the elements of movement in peripheral cytoplasm are: force generation by actin and myosin; orientation of force by controlled gel-sol transformations; regulation of these transformations by ionized calcium.