Characterisation of RPGR and RPGR-IP in the ciliary network

Cilia are antennae-like structures at the cell surface having important roles in cell motility, fluid movement as well as sensory functions. Defects in ciliogenesis or ciliary function give rise to a variety of human disorders (ciliopathies).

Some of the main players in the ciliary protein network seem to be the Retinitis Pigmentosa GTPase regulator (RPGR) and the RPGR-interacting proteins RPGR-IP1 and RPGR-IP1L. The RPGR gene encodes multiple protein isoforms sharing an N-terminal domain homologous to the regulator of chromosome condensation 1 (RCC1), a guanine exchange factor (GEF) for the RanGTPase. Mutations of RPGR lead to Retinitis Pigmentosa, a disease with progressive degeneration of retinal photoreceptors. RPGR-IP1 and the homologous protein RPGR-IP1 like consist of four domains: an N-terminal coiled-coil domain, two C2 domains (C2-N and C2-C) and a C-terminal RPGR-interacting domain (RID). Mutations in RPGR-IP1 cause Leber´s Congenital Amaurosis, while mutations in the RPGR-IP1L gene are associated with the more severe Meckel-Gruber and Joubert syndromes.

The molecular mechanisms underlying these disorders and the exact role and composition of the different protein complexes remain however largely unknown. To address these questions we perform a detailed biochemical and structural analysis of the individual components and of the protein complexes. Pulldown experiments, fluorescence polarization assays and gel permeation chromatography revealed an interaction of RPGR with the prenyl-binding protein PDEd and with RPGR-IP. Furthermore, a ternary complex consisting of RPGR and PDEd with either Rheb or Arl2/3 can be formed. However, the precise function of these complexes remains elusive.