USP9X counteracts differential ubiquitination of NPHP5 by MARCH7 and BBS11 to regulate ciliogenesis

by Arindam Das, Jin Qian, William Y. Tsang

Ciliogenesis is a fundamental biological process central to human health. Precisely how this process is coordinated with the cell cycle remains an open question. We report that nephrocystin-5 (NPHP5/IQCB1), a positive regulator of ciliogenesis, is a stable and low turnover protein subjected to cycles of ubiquitination and deubiquitination. NPHP5 directly binds to a deubiquitinating enzyme USP9X/FAM and two E3 ubiquitin ligases BBS11/TRIM32 and MARCH7/axotrophin. NPHP5 undergoes K63 ubiquitination in a cell cycle dependent manner and K48/K63 ubiquitination upon USP9X depletion or inhibition. In the G0/G1/S phase, a pool of cytoplasmic USP9X recruited to the centrosome by NPHP5 protects NPHP5 from ubiquitination, thus favouring cilia assembly. In the G2/M phase, USP9X dissociation from the centrosome allows BBS11 to K63 ubiquitinate NPHP5 which triggers protein delocalization and loss of cilia. BBS11 is a resident centrosomal protein, whereas cytoplasmic USP9X sequesters the majority of MARCH7 away from the centrosome during interphase. Depletion or inhibition of USP9X leads to an accumulation of centrosomal MARCH7 which K48 ubiquitinates NPHP5, triggering protein degradation and cilia loss. At the same time, BBS11 K63 ubiquitinates NPHP5. Our data suggest that dynamic ubiquitination and deubiquitination of NPHP5 plays a crucial role in the regulation of ciliogenesis.

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