Fig. 13. A proposed model for quinine-induced protrusion formation. (1) The plasma membrane (thin line) is lined with a cortical layer of actin and myosin (thick line) which produces strong cortical tension. (2) The contractile vacuole (CV) fuses with the plasma membrane. (3) High inner pressure due to the elevated cortical tension forces the vacuolar membrane out through the hole of the cortical layer to form a protrusion. (4) Contraction of the cortical layer continues to force the cytosol into the protrusion. The protrusion expands because of the low tension of its membrane which lacks a cortical layer of F-actin. (5) A cortical layer of F-actin (grey line) extends from the cell body into the protrusion along its lateral membrane, but the absence of F-actin cortex at its distal region allows its further elongation. (6) The actomyosin layer continues to contract, pushing the cellular content forwards into the protrusion. This results in locomotion of the cell. The cortical layer of F-actin on the lateral side of the protrusion continues to elongate forwards, but the apex of the protrusion remains virtually free of F-actin or is only loosely bound to a thin actin layer (thin grey line), allowing sustained movement of the cell. (7) Forward movement of the cell continues if the actomyosin cortex at the back of the cell continues to supply forces necessary for elongation of the protrusion and if the thin actin layer continues to be detached from the membrane. Release of actin, myosin II, and other cortex-associated molecules from the cortical cytoskeleton in the tail region by a mechanism probably involving the regulation by F-actin-crosslinking proteins (Jason and Taylor, 1993) is also required for their reuse in the front. A new actin layer starts to be formed inside the apical membrane immediately after detachment of the existing actin layer, while the detached actin layer moves backwards due to contraction of the cell cortex and is gradually disintegrated (dashed grey line). The detachment of the actin layer at the tip of the protrusion may be discrete events or continuous process forming a spiral sheet of F-actin as illustrated here. (8) If the plasma membrane of the entire protrusion becomes firmly attached to the F-actin cortex, the protrusion stops extending. A layer of myosin II gradually extends from the cell body towards the leading edge. (9) The entire protrusion becomes lined with the myosin layer and the protrusion retracts.
