Primary cilia regulate epithelial differentiation and organ function. Failure of mutant polycystins to localize to cilia abolishes flow-stimulated calcium signaling and causes autosomal dominant polycystic kidney disease. We identify a conserved amino acid sequence， KVHPSST， in the C-terminus of polycystin-1 (PC1) that serves as a ciliary-targeting signal. PC1 binds a multimeric protein complex consisting of several GTPases (Arf4， Rab6， Rab11) and the GTPase-activating protein (GAP)， ArfGAP with SH3 domain， ankyrin repeat and PH domain 1 (ASAP1) in the Golgi， which facilitates vesicle budding and Golgi exocytosis. A related N-terminal ciliary-targeting sequence in polycystin-2 similarly binds Arf4... More
Primary cilia regulate epithelial differentiation and organ function. Failure of mutant polycystins to localize to cilia abolishes flow-stimulated calcium signaling and causes autosomal dominant polycystic kidney disease. We identify a conserved amino acid sequence， KVHPSST， in the C-terminus of polycystin-1 (PC1) that serves as a ciliary-targeting signal. PC1 binds a multimeric protein complex consisting of several GTPases (Arf4， Rab6， Rab11) and the GTPase-activating protein (GAP)， ArfGAP with SH3 domain， ankyrin repeat and PH domain 1 (ASAP1) in the Golgi， which facilitates vesicle budding and Golgi exocytosis. A related N-terminal ciliary-targeting sequence in polycystin-2 similarly binds Arf4. Deletion of the extreme C-terminus of PC1 ablates Arf4 and ASAP1 binding and prevents ciliary localization of an integral membrane CD16.7-PC1 chimera. Interactions are confirmed for chimeric and endogenous proteins through quantitated in vitro and cell-based approaches. PC1 also complexes with Rab8; knockdown of trafficking regulators Arf4 or Rab8 functionally blocks CD16.7-PC1 trafficking to cilia. Mutations in rhodopsin disrupt a similar signal and cause retinitis pigmentosa， while Bardet-Biedl syndrome， primary open-angle glaucoma， and tumor cell invasiveness are linked to dysregulation of ASAP1 or Rab8 or its effectors. In this paper， we provide evidence for a conserved GTPase-dependent ciliary-trafficking mechanism that is shared between epithelia and neurons， and is essential in ciliary-trafficking and cell homeostasis.
