TMEM67 is required for the gating function of the transition zone that controls entry of membrane-associated proteins ARL13B and INPP5E into primary cilia.

Primary cilia transduce signals via transmembrane and membrane-associated proteins localized to the ciliary membrane in vertebrate cells. In humans, transmembrane protein 67 (TMEM67), a component of the multiprotein complex functioning as a gatekeeper at the transition zone (TZ) of primary cilia, is mutated in patients suffering from cilia-related pleiotropic diseases, collectively referred to as ciliopathies. The requirement of TMEM67 for the gating function of the TZ that delivers membrane proteins into the ciliary compartment has not been determined. In this study, we established hTERT-RPE1 cells with knockout (KO) of TMEM67 and examined whether cilium formation and TZ gating are affected by its ablation. TMEM67-KO cells displayed impaired ciliogenesis, elongated cilia, perturbed ciliary localization of membrane-associated proteins ARL13B and INPP5E but normal recruitment of TZ proteins CEP290, RPGRIP1L and NPHP5. The exogenous expression of ciliopathy-associated TMEM67 mutants restored ciliary localization of ARL13B and INPP5E but failed to attenuate aberrant cilium elongation in TMEM67-KO cells. Furthermore, we found that TMEM67 localization is not confined to the TZ but extends into the cilium. Our findings indicate that TMEM67 is required not only for ciliogenesis and cilium length regulation but also for the gating function of the TZ independently of RPGRIP1L/CEP290/NPHP5 recruitment to this region. They further suggest that aberrant cilium elongation underlies the pathogenesis of TMEM67-linked ciliopathies.