Fig. 11. Model for the breakdown of the modular actin bundles that support Drosophila bristles. (a) Two adjacent modules (gray boxes) are shown attached to the plasma membrane (vertical black line) of the cell. The orientation of polarized actin filaments in the modules and the orientation of the modules relative to the bristle tip are shown to the right. In these panels, only the lower module undergoes breakdown, whereas the upper module remains intact for comparison. (b) Actin filaments shorten by the loss of subunits from their barbed ends. This process can be accelerated by inhibiting protein synthesis with cycloheximide. (c) The module also begins to breakdown into submodules by longitudinal cleavages. This process can be accelerated by inhibiting actin polymerization with cytochalasin. It is not clear whether depolymerization or subbundle formation is initiated sequentially or simultaneously. (d) The module is completely split from the plasma membrane, leaving behind a membrane-bound actin `ghost'. Some submodules can become completely split off from the module proper and can be found deeper in the cytoplasm. (e) In the end, only the `ghost' of the module remains attached to the membrane.