ARHGAP18 complexes with both YAP and Merlin and is required for basal actin bundles

The organization of the cell's cytoskeletal filaments is coordinated through a complex network of signaling cascades activated by both internal and external cues. Two major actin regulatory pathways are signal transduction through Rho family GTPases and growth and proliferation signaling through the Hippo pathway. These two pathways define the actin cytoskeleton, controlling foundational cellular attributes such as morphology and the organization of actin-based structures, and are hijacked to promote proliferation and motility in aggressive cancers. In this study, we use human epithelial cells to investigate the interplay between the Hippo and Rho Family signaling pathways. We identify that the RhoA GTPase-activating protein, ARHGAP18, forms a complex with two Hippo pathway components, the tumor suppressor Merlin (NF2), and the transcriptional coactivator YAP. Using super-resolution STORM microscopy, we characterize single-filament-level changes in the actin cytoskeleton that arise from CRISPR/CAS9 knockout of ARHGAP18. We report that the loss of ARHGAP18 results in cytoskeletal alterations associated with dysregulation of RhoA signaling at apical structures and aberrant nuclear localization of YAP. These findings provide additional support for models suggesting that Hippo and Rho family GTPase signaling cascades may be temporally and spatially coordinated in the regulation of the actin cytoskeleton.