Identification of a Candidate Rad1 Subunit for the Kinetoplastid 9-1-1 (Rad9-Hus1-Rad1) Complex

The dynamic subcellular localisation of Rad1 is cell cycle dependent in Leishmania major

The subcellular localisation of Rad1, a subunit of the Leishmania major 9-1-1 complex, remains unexplored. Herein, we reveal that Rad1 localises predominantly to the nucleus. Upon hydroxyurea treatment, the diffuse nuclear localisation of Rad1 becomes more punctate, suggesting that Rad1 is responsive to replication stress. Moreover, Rad1 localisation correlates with cell cycle progression. In the majority of G1 to early S-phase cells, Rad1 localises predominantly to the nucleus. As cells progress from late-S phase to mitosis, Rad1 relocalizes to both the nucleus and the cytoplasm in ∼90 % of cells. This pattern of distribution is different from Rad9 and Hus1, which remain nuclear throughout the cell cycle, suggesting Leishmania Rad1 may regulate 9-1-1 activities and/or perform relevant functions outside the 9-1-1 complex.

Functional compartmentalization of Rad9 and Hus1 reveals diverse assembly of the 9-1-1 complex components during the DNA damage response in Leishmania

The Rad9‐Rad1‐Hus1 (9‐1‐1) complex is a key component in the coordination of DNA damage sensing, cell cycle progression and DNA repair pathways in eukaryotic cells. This PCNA‐related trimer is loaded onto RPA‐coated single stranded DNA and interacts with ATR kinase to mediate effective checkpoint signaling to halt the cell cycle and to promote DNA repair. Beyond these core activities, mounting evidence suggests that a broader range of functions can be provided by 9‐1‐1 structural diversification. The protozoan parasite Leishmania is an early‐branching eukaryote with a remarkably plastic genome, which hints at peculiar genome maintenance mechanisms. Here, we investigated the existence of homologs of the 9‐1‐1 complex subunits in L. major and found that LmRad9 and LmRad1 associate with chromatin in response to replication stress and form a complex in vivo with LmHus1. Similar to LmHus1, LmRad9 participates in telomere homeostasis and in the response to both replication stress and double strand breaks. However, LmRad9 and LmHus1‐deficient cells present markedly opposite phenotypes, which suggest their functional compartmentalization. We show that some of the cellular pool of LmRad9 forms an alternative complex and that some of LmHus1 exists as a monomer. We propose that the diverse assembly of the Leishmania 9‐1‐1 subunits mediates functional compartmentalization, which has a direct impact on the response to genotoxic stress.