Mec1-dependent phosphorylation of Mms21 modulates its SUMO ligase activity

• Mms21 was phosphorylated in the C-terminus in a Mec1-dependent manner.
• Phosphoablative mutants mms21-S260IS261I/S260AS61A were hypersensitive to MMS.
• Phosphorylation positively regulated Mms21 SUMO ligase activity.
• mms21-S260IS261I was prone to chromosome loss after DNA damage.

The SUMO ligase Mms21, which is a subunit of the Smc5/6 complex, is required for DNA repair. Here we present results showing that Mms21 was phosophorylated during S-phase in a manner dependent on the DNA damage kinase Mec1. Phosphorylation of Mms21 occurred in unchallenged cells, but was more abundant in the presence of DNA damaging agents. Mass spectrometry identified five phosphorylated serines organized in two regions of Mms21, and two C-terminal serines, S260 and S261, formed part of a Mec1/Tel1 consensus motif. Nonphosphorylatable substitutions of the C-terminal serines, inactivation of Mec1 or removal of the Mms21 C-terminus all abolished Mms21 phosphorylation. Additionally, strains carrying Mms21 phosphoablative alleles displayed reduced SUMO ligase activity, sensitivity to MMS and an increased rate of chromosome loss in the presence of MMS. We propose that one function of S260 S261 phosphorylation is to positively regulate the SUMO ligase activity of Mms21 and thereby promote genomic stability.