The human RPGRIP1L gene is one of the causal genes in Meckel and Joubert type B syndromes, two autosomal-recessive multisystem ciliopathies []. RPGRIP1L (also called MKS5, NPHP-8, and for the mouse gene, Ftm, Fantom) is required from developmental processes such as the establishment of left-right asymmetry and patterning of the neural tube and the limbs [], and ventricular septal development []. The protein is mainly found at the ciliary transition zone, where it forms a complex with nephrocystin-1 and nephrocystin-4 [, ]. RPGRIP1L is essential for hair follicle morphogenesis []and for planar cell polarity In the mouse cochlea and in the zebrafish floor plate [].
RPGR-interacting protein 1 (RPGRIP1) is mutated in the eye disease Leber congenital amaurosis (LCA) and its structural homologue, RPGRIP1-like (RPGRIP1L, also called NPHP8 or fantom), is mutated in many different ciliopathies [, ]. Both are multidomain proteins that are predicted to interact with retinitis pigmentosa G-protein regulator (RPGR) []. Both consist of an N-terminal coiled coil domain, two C2 domains (C2N and C2C), and a C-terminal RPGR-interacting domain (RID). RID is a C2 domain with a canonical beta sandwich structure that does not bind Ca2+ and/or phospholipids and thus constitutes a unique type of protein-protein interaction module [].Both RPGRIP1 and RPGRIP1L interact with the ciliary transition zone protein nephrocystin 4 (NPHP4) via their C2C domain [, ]. An hypothesis is that RPGRIP1 and RPGRIP1L function as cilium-specific scaffolds that recruit a Nek4 signaling network which regulates cilium stability []. The expression of RPGRIP1 seems to be limited to photoreceptors and amacrine cells in the retina [], whereas RPGRIP1L is found in other tissues as well.
