Actin filament turnover removes bundles from Drosophila bristle cells

Drosophila bristle cells form enormous extensions that are supported by equally impressive scaffolds of modular, polarized and crosslinked actin filament bundles. As the cell matures and support is taken over by the secreted cuticle, the actin scaffold is completely removed. This removal begins during cell elongation and proceeds via an orderly series of steps that operate on each module. Using confocal and electron microscopy, we found that the approximately 500-filament modules are fractured longitudinally into 25-50-filament subbundles, indicating that module breakdown is the reverse of assembly. Time-lapse confocal analysis of GFP-decorated bundles in live cells showed that modules were shortened by subunit removal from filament barbed ends, again indicating that module breakdown is the reverse of assembly. Module shortening takes place at a fairly slow rate of approximately 1microm/hour, implying that maximally crosslinked modules are not rapidly depolymerized. Barbed-end depolymerization was prevented with jasplakinolide and accelerated with cycloheximide, indicating that barbed-end maintenance requires continuous protein synthesis. Subbundle adhesion was lost in the presence of cytochalasin, indicating that continuous actin polymerization is required. Thus, these polarized actin filament bundles are dynamic structures that require continuous maintenance owing to protein and actin filament turnover. We propose that after cell elongation, maintenance falls behind turnover, resulting in the removal of this modular cytoskeleton.