Manuscript submitted to eLife

Intraflagellar transport (IFT) is a highly conserved mechanism for motor-driven 12 transport of cargo within cilia, but how this cargo is selectively transported to cilia and across the 13 diffusion barrier is unclear. WDR35/IFT121 is a component of the IFT-A complex best known for 14 its role in ciliary retrograde transport. In the absence of WDR35, small mutant cilia form but fail to 15 enrich in diverse classes of ciliary membrane proteins. In Wdr35mouse mutants, the IFT-A 16 peripheral components are degraded and core components accumulate at the transition zone. 17 We reveal deep sequence homology and structural similarity of WDR35 and other IFT-As to the 18 coatomer COPI proteins and ’, and demonstrate an accumulation of ‘coat-less’ vesicles which 19 fail to fuse with Wdr35mutant cilia. Our data provides the first in situ evidence of a novel 20 coatomer function for WDR35 likely with other IFT-A proteins in delivering ciliary membrane 21 cargo from the Golgi necessary for cilia elongation. 22