DYNAMICS OF THE CONTRACTILE SYSTEM IN THE PSEUDOPODIAL TIPS OF NORMALLY LOCOMOTING AMEBAS, DEMONSTRATED INVIVO BY VIDEO-ENHANCEMENT

Behaviour of the membrane and contractile system was directly recorded in the advancing and retracting frontal zones of spontaneously locomoting or stimulated amoebae. The advancing pseudopodial tips alternately slow down and accelerate. In the slowing phase the frontal hyaline caps are flat and compressed by countercontraction of the cortical actin network beneath the leading edge. At this stage the membrane-cytoskeleton complex splits: the detached contractile layer is retracted inwards, and the membrane lifted outwards. The fluid endoplasm fraction is filtered forward through the detached actin network. This results in a local hydrostatic pressure drop, immediately restores the forward flow of endoplasm and initiates the acceleration phase of the leading edge progression. The frontal membrane, temporarily disconnected from the cytoskeletal layer, is free to slide and extend forward, but the new submembrane contractile network is soon repolymerized. In this way, after making one step forward, the frontal zone recovers its former state, and the cycle is then repeated. The cortex disassembly-reassembly cycles at the leading edge are produced every 2 s, on average. Retraction of the frontal contractile layers is part of the general centripetal cortex flow observed during motor functions of amoebae and many other cells, and is therefore associated with various other backward movements observed within and on the surface of advancing frontal zones of amoebae. The backward movement of the contractile cortex is also responsible for the withdrawal of previously advancing pseudopodia, if the detachment of successive contractile sheets from the frontal membrane ceases. It was demonstrated that the action of attractants and repellents is based on the activation or inhibition, respectively, of rhythmic disassembly of the membrane-cytoskeleton complex at the leading edge. © 1990 Springer-Verlag.