ARMC5 has been identified as the gene responsible for PBMAH (primary bilateral macronodular adrenal hyperplasia). ARMC5 inactivating mutations are reported in 20 to 25% of PBMAH patients. ARMC5 is considered as a tumor suppressor gene controlling apoptosis and regulating steroidogenesis. The mechanisms of action of ARMC5 are unknown. The structure of the ARMC5 protein contains ARM repeats and a BTB domain, patterns known to play a role in protein-protein interactions. Therefore identification of proteins that interact with ARMC5 and study of the mechanisms of this interaction will help to understand its function. By co-immunoprecipitation followed by mass spectrometry in HEK293 cells we identified a potential interaction between ARMC5 and Cullin3 (Cul3), also suggested in online databases and by 2 Hybrid Assay (Hu et al., 2017). Cul3 is a protein that mediates the ubiquitination process and subsequent degradation of specific protein substrates. Therefore, the aim of this study was to confirm this interaction and to investigate its mechanisms.

We used immunoprecipitation experiments with HA-tagged Cul3 and the bioluminescence resonance energy transfer (BRET) proximity assay in HEK293 cells in order to confirm and investigate the interaction of ARMC5 with Cul3.

ARMC5 co-immunoprecipitated with HA-Cul3 and a hyperbolic BRET saturation curve was observed with YFP-Cul3 and ARMC5-Luc indicating a specific close proximity between these two proteins. We have also observed that a missense mutation in the BTB domain (p.L754P) of ARMC5 disrupts the interaction with Cul3. Altogether, these complementary approaches demonstrate that ARMC5 and Cul3 form a complex involving the BTB domain of ARMC5.

These data demonstrate that Cul3 is an ARMC5 partner. A likely direct interaction involves the BTB domain of ARMC5 and can be altered by pathogenic ARMC5 missense mutations. This suggests that ARMC5 participates in the ubiquitination process and open new perspectives in the pathophysiology of PBMAH.