Tectonic Complex Impedes Diffusion through the Ciliary Transition Zone to Ensure Proper Sorting of Membrane Proteins

The primary cilium is a signaling organelle with a unique membrane composition maintained by a diffusional barrier residing at the transition zone. Many transition zone proteins, such as the tectonic complex, are linked to preserving ciliary composition; however, whether these proteins regulate active transport or physically impede membrane diffusion remains unknown. To understand tectonic’s role, we generated a photoreceptor specific Tctn1 knockout mouse. Loss of Tctn1 resulted in an absence of the entire tectonic complex yet had minimal effects on transition zone structure. Interestingly, we found that protein composition of the photoreceptor cilium was disrupted as non-resident membrane proteins accumulated in the cilium over time, ultimately resulting in photoreceptor degeneration. We further show that membrane proteins moved faster through the transition zone illustrating that the tectonic complex acts as a physical barrier to slow diffusion of membrane proteins so they can be properly sorted by ciliary transport carriers.