Phosphorylation by CK2 regulates MUS81/EME1 in mitosis and after replication stress

The MUS81complex is crucial for preserving genome stability through the resolution of branched DNA intermediates in mitosis. However, untimely activation of the MUS81complex in S-phaseis dangerous. Little is known about the regulation of the human MUS81complex and how deregulated activation affects chromosome integrity. Here, we show that the CK2 kinase phosphorylates MUS81at Serine 87 in late-G2/ mitosis, and upon mild replication stress. Phosphorylated MUS81interacts with SLX4, and this association promotes the function of the MUS81complex. In line with a role in mitosis, phosphorylation at Serine 87 is suppressed in S-phaseand is mainly detected in the MUS81molecules associated with EME1. Loss of CK2-dependent MUS81phosphorylation contributes modestly to chromosome integrity, however, expression of the phosphomimic form induces DSBs accumulation in S-phase, because of unscheduled targeting of HJ-like DNA intermediates, and generates a wide chromosome instabilityphenotype. Collectively, our findings describe a novel regulatory mechanism controlling the MUS81 complex functionin human cells. Furthermore, they indicate that, genome stability depends mainly on the ability of cells to counteract targeting of branched intermediates by the MUS81/EME1 complex in S-phase, rather than on a correct MUS81function in mitosis.