The subunits of the S-phase checkpoint complex Mrc1/Tof1/Csm3: dynamics and interdependence.
BACKGROUND
The S-phase checkpoint aims to prevent cells from generation of extensive single-stranded DNA that predisposes to genome instability. The S. cerevisiae complex Tof1/Csm3/Mrc1 acts to restrain the replicative MCM helicase when DNA synthesis is prohibited. Keeping the replication machinery intact allows restart of the replication fork when the block is relieved. Although the subunits of the Tof1/Csm3/Mrc1 complex are well studied, the impact of every single subunit on the triple complex formation and function needs to be established.


RESULTS
This work studies the cellular localization and the chromatin binding of GFP-tagged subunits when the complex is intact and when a subunit is missing. We demonstrate that the complex is formed in cell nucleus, not the cytoplasm, as Tof1, Csm3 and Mrc1 enter the nucleus independently from one another. Via in situ chromatin binding assay we show that a Tof1-Csm3 dimer formation and chromatin binding is required to ensure the attachment of Mrc1 to chromatin. Our study indicates that the translocation into the nucleus is not the process to regulate the timing of chromatin association of Mrc1. We also studied the nuclear behavior of Mrc1 subunit in the process of adaptation to the presence hydroxyurea. Our results indicate that after prolonged HU incubation, cells bypass the S-phase checkpoint and proceed throughout the cell cycle. This process is accompanied by Mrc1 chromatin detachment and Rad53 dephosphorylation.


CONCLUSIONS
In S. cerevisiae the subunits of the S-phase checkpoint complex Mrc1/Tof1/Csm3 independently enter the cell nucleus, where a Tof1-Csm3 dimer is formed to ensure the chromatin binding of Mrc1 and favor DNA replication and S-phase checkpoint fork arrest. In the process of adaptation to the presence of hydroxyurea Mrc1 is detached from chromatin and Rad53 checkpoint activity is diminished in order to allow S-phase checkpoint escape and completion of the cell cycle.
Institute of Molecular Biology "Roumen Tsanev", Bulgarian Academy of Sciences, 21 "Acad. George Bonchev" Str., 1113 Sofia, Bulgaria.
Sonya Dimitrova
SD
Uzunova
Institute of Molecular Biology "Roumen Tsanev", Bulgarian Academy of Sciences, 21 "Acad. George Bonchev" Str., 1113 Sofia, Bulgaria.
Alexander Stefanov
AS
Zarkov
Institute of Molecular Biology "Roumen Tsanev", Bulgarian Academy of Sciences, 21 "Acad. George Bonchev" Str., 1113 Sofia, Bulgaria.
Anna Marianova
AM
Ivanova
Institute of Molecular Biology "Roumen Tsanev", Bulgarian Academy of Sciences, 21 "Acad. George Bonchev" Str., 1113 Sofia, Bulgaria.
Stoyno Stefanov
SS
Stoynov
Institute of Molecular Biology "Roumen Tsanev", Bulgarian Academy of Sciences, 21 "Acad. George Bonchev" Str., 1113 Sofia, Bulgaria.
Marina Nedelcheva
MN
Nedelcheva-Veleva
Adaptation; Cell cycle; Csm3; In situ chromatin binding assay; Mrc1; Protein localization; Tof1