Molecular basis for the activation of outer dynein arms in cilia.

Multiciliogenesis requires large-scale biosynthesis of motility-powering axonemal inner and outer dynein arm motors (IDAs and ODAs) before their intraflagellar transport (IFT) into cilia. ODAs are inhibited by the packaging chaperone Shulin during ciliogenesis in Tetrahymena thermophila. How Shulin is released for ODAs to become active inside cilia remains unclear. Here we uncover a molecular mechanism for ODA activation. We establish interactions between DNAAF9 (human Shulin) and mammalian ODA subunits, IFT proteins and the ciliary small guanosine triphosphatase (GTPase) ARL3 using proteomics and in vitro reconstitutions. Mutagenesis combined with biochemical and structural studies reveal that DNAAF9 and Shulin preferentially bind active Arl3-GTP highlighting a cross-species conservation of this interaction. GTP-loaded Arl3 can access, bind and displace Shulin from the packaged ODA-Shulin complex. We propose that, once the inhibited ODA complex enters growing cilia, Arl3-GTP displaces Shulin (DNAAF9) and sequesters it away from ODAs, promoting activation of their motility specifically inside cilia.