Building the primary cilium membrane

摘要

Ciliogenesis involves coordinated assembly of a microtubule-based axoneme from the mother centriole andvesicular membrane transport and fusion forming a ciliarymembrane around the developing axoneme. We,and others have reported that a Rab11-Rab8 cascadefunctions in ciliogenesis. Using live high-resolutionfluorescence microscopy imaging we show that ciliarymembrane assembly proceeds following Rabin8 (a Rab8activator) binding to Rab11 membranes. Rabin8 transportvia Rab11 vesicles to the centrosome is observedresulting in localized activation of Rab8 and leads to initiationof ciliary membrane assembly. Using proteomicsapproaches, we have discovered that Rabin8 binds to theTRAPPII tethering complex and find that this interactionis important for Rabin8 centrosomal targeting duringciliogenesis. Our work suggests that Rabin8 membranetransport is a highly regulated process controlled byserum-dependent and serum-independent signaling. Interestingly,following ciliogenesis Rabin8 centrosomal localizationis lost resulting in reduced Rab8 activation at theciliary membrane. This finding along with a previousreport describing Rabin8 association with Bardet-Biedlsyndrome (BBS) proteins has led us to hypothesize thatregulation of centrosomal Rabin8 levels is important forestablishing the length of primary cilium, an importantfactor in ciliary signaling. Finally, we describe the discoveryof additional factors associated with the Rab11-Rab8 traffickingpathway that function in organizing membranestructure during ciliogenesis.