The polarity protein Inturned links NPHP4 to Daam1 to control the subapical actin network in multiciliated cells

Motile cilia polarization requires intracellular anchorage to the cytoskeleton; however, the molecular machinery that supports this process remains elusive. We report that Inturned plays a central role in coordinating the interaction between cilia-associated proteins and actin-nucleation factors. We observed that knockdown ofnphp4in multiciliated cells of theXenopus laevisepidermis compromised ciliogenesis and directional fluid flow. Depletion ofnphp4disrupted the subapical actin layer. Comparison to the structural defects caused byinturneddepletion revealed striking similarities. Furthermore, coimmunoprecipitation assays demonstrated that the two proteins interact with each other and that Inturned mediates the formation of ternary protein complexes between NPHP4 and DAAM1. Knockdown ofdaam1, but notformin-2, resulted in similar disruption of the subapical actin web, whereasnphp4depletion prevented the association of Inturned with the basal bodies. Thus, Inturned appears to function as an adaptor protein that couples cilia-associated molecules to actin-modifying proteins to rearrange the local actin cytoskeleton.