A role for the BBSome in restricting the entry of non-ciliary proteins into cilia

Cilia possess the inherent ability to restrict the entry of non-ciliary components through the ciliary gate. Mutations in genes encoding components of the ciliary gate result in various cilia-related diseases known as ciliopathies. Yet, how non-ciliary residents are prevented from accumulating in cilia remains unclear. Here, we observe ciliary accumulation of non-ciliary proteins in BBSome-deficient mutants (Bardet-Biedl syndrome), a phenotype reminiscent of ciliary gate-defective mutants such as mks-5 and nphp-4 . Time-lapse imaging shows that these non-ciliary proteins move bidirectionally along cilia at IFT velocities, and bimolecular fluorescence complementation (BiFC) reveals that the non-ciliary protein ELMD-1 associates with the IFT component IFT-43 in BBSome-deficient mutants, indicating that they are mis-loaded onto IFT trains. This misloading allows them to bypass the ciliary gate. Fluorescence recovery after photobleaching (FRAP) suggests the gate remains largely functional, supporting a potential two-step model of ciliary composition control: cargo selection/loading and ciliary gating. We propose that defects in cargo selection/loading (and potentially export) alter ciliary composition, allowing unintended protein entry.