Microtubules coordinate with actin assembly and disassembly to elongate cortical actin cables

The coordination of actin filament elongation, bundling and turnover within bundled actin structures is poorly understood. Nurse cells extend bundles of actin filaments (cables) from the cortex during Drosophila oogenesis. We previously showed that cable assembly requires a developmentally-regulated microtubule (MT) network. Here we demonstrate that MTs coordinate with actin filament assembly and disassembly to promote the persistent elongation of actin cables. Disrupting the MT network caused actin cables to detach from the cortex and become free in the cytoplasm. The subsequent fate of those free cables, shrinking over time or maintaining a stable length, depended on the MT manipulation. Furthermore, maintaining cortical continuity and persistent cable elongation requires a regulated imbalance between filament assembly and disassembly. Favoring disassembly caused cable detachment and shrinkage while reducing disassembly allowed free cables to continue elongating in the cytoplasm. Our results also suggest that both filament assembly and disassembly occur at the cortical filament barbed-ends protected from excessive disassembly by Capping Protein.