From Lowe syndrome to Dent disease: correlations between mutations of the OCRL1 gene and clinical and biochemical phenotypes

Recognition of the F&H motif by the Lowe syndrome protein OCRL

Lowe syndrome and Type 2 Dent disease are caused by defects in the inositol 5-phosphatase OCRL. Most missense mutations in the OCRL ASH-RhoGAP domain found in affected patients abolish interactions with the endocytic adaptors APPL1 and Ses (both Ses1 and Ses2), which bind OCRL through a short F&H motif. Using X-ray crystallography, we have identified the F&H motif binding site on the RhoGAP domain of OCRL. We further show that clinical mutations affect F&H binding indirectly by destabilizing the RhoGAP fold. In contrast, a clinical mutation that does not perturb F&H binding and ASH-RhoGAP stability disrupts OCRL's interaction with Rab5. Additionally, OCRL's F&H binding site is conserved even in species that do not express APPL or Ses. Our study predicts the existence of other OCRL binding partners and demonstrates the critical role of the perturbation of OCRL interactions in disease.

A structural basis for Lowe syndrome caused by mutations in the Rab-binding domain of OCRL1

The oculocerebrorenal syndrome of Lowe (OCRL), also called Lowe syndrome, is characterized by defects of the nervous system, the eye and the kidney. Lowe syndrome is a monogenetic X-linked disease caused by mutations of the inositol-5-phosphatase OCRL1. OCRL1 is a membrane-bound protein recruited to membranes via interaction with a variety of Rab proteins. The structural and kinetic basis of OCRL1 for the recognition of several Rab proteins is unknown. In this study, we report the crystal structure of the Rab-binding domain (RBD) of OCRL1 in complex with Rab8a and the kinetic binding analysis of OCRL1 with several Rab GTPases (Rab1b, Rab5a, Rab6a and Rab8a). In contrast to other effectors that bind their respective Rab predominantly via α-helical structure elements, the Rab-binding interface of OCRL1 consists mainly of the IgG-like β-strand structure of the ASPM-SPD-2-Hydin domain as well as one α-helix. Our results give a deeper structural understanding of disease-causing mutations of OCRL1 affecting Rab binding.