EXOC6B promotes cilial elongation via autophagy-dependent protein turnover

Exocyst Complex Component 6b (EXOC6B), a constituent of highly conserved octameric exocyst complex, plays a pivotal role in tethering and spatial targeting of post-Golgi vesicles to the plasma membrane, thereby facilitating exocytosis. Mutations in the EXOC6B gene has been associated with severe neurological and skeletal disorders, such as spondyloepimetaphyseal dysplasia with joint laxity type 3 (SEMDJL3) and Laurin-Sandrow Syndrome, highlighting its critical role in the development. SEMDJL3 patient fibroblasts showed impaired ciliogenesis. Thus, in this study, we investigated the functional importance of EXOC6B in cilium formation and the underlying molecular mechanisms. Here, we observed that cilia were shorter in EXOC6B knockout HEK 293T cells. On the other hand, forced expression of EXOC6B in HEK 293T cells augmented cilia length. Mechanistic analysis of EXOC6B-mediated cilia effects revealed that EXOC6B enhanced basal autophagy through the activation of AMPK and inhibition of mTOR signalling pathways. Analysis of clinically reported mutations in EXOC6B gene showed that the region beyond amino acid Tyr³⁰² is critical for interaction with ciliary proteins, induction of basal autophagy and cilia length enhancement. Taken together, we show that EXOC6B promotes cilia length through the activation of basal autophagy.