Human hRad1 but not hRad9 protects hHus1 from ubiquitin-proteasomal degradation

Three of the Rad family proteins, Rad9, Rad1, and Hus1, can interact with each other and form a heterotrimeric complex that is thought to play a role in the sensing step of the DNA integrity checkpoint pathways, but the nature of the Rad9-Rad1-Hus1 complex assembly remains enigmatic. Here, we demonstrate that the human hRad1 protein plays a significant role as molecular chaperone in the process of the hRad9-hRad1-hHus1 heterotrimeric complex formation. In contrast to hRad1, hHus1 is an unstable protein that is actively degraded via the ubiquitin-proteasome pathway. We show that treating cells with proteasome-specific inhibitors stabilizes hHus1 expression. Moreover, hRad1 can associate with hHus1 in the absence of hRad9 and protect hHus1 from ubiquitination and degradation in the cytoplasm. Importantly, genotoxic stress induces hRad1 expression and stabilizes the hHus1 protein. Taken together, these findings suggest a novel role of hRad1 as a potential intrinsic chaperone in the stabilization of hHus1 for the hRad9-hRad1-hHus1 checkpoint complex formation.