Noncovalent SUMO-1 binding activity of thymine DNA glycosylase (TDG) is required for its SUMO-1 modification and colocalization with the promyelocytic leukemia protein

SUMO-1 is a member of a family of ubiquitin-like molecules that are post-translationally conjugated to various cellular proteins in a process that is mechanistically similar to ubiquitylation. To identify molecules that bind noncovalently to SUMO-1, we performed yeast two-hybrid screening with a SUMO-1 mutant that cannot be conjugated to target proteins as the bait. This screening resulted in the isolation of cDNAs encoding the b isoform of thymine DNA glycosylase (TDGb). A deletion mutant of TDGb (TDGb(Δ11)) that lacks a region shown to be required for noncovalent binding of SUMO-1 was also found not to be susceptible to SUMO-1 conjugation at an acljacent lysine residue, suggesting that such binding is required for covalent modification. In contrast, another mutant of TDGb (TDGb(KR)) in which the lysine residue targeted for SUMO-1 conjugation is replaced with arginine retained the ability to bind SUMO-1 noncovalently. TDGb was shown to interact with the promyelocytic leukemia protein (PML) in vitro as well as to colocalize with this protein to nuclear bodies in transfected cells. TDGb(KR) also colocalized with PML, whereas TDGb(Δ11) did not, indicating that the noncovalent SUMO-1 binding activity of TDGb is required for colocalization with PML. Furthermore, SUMO-1 modification of TDGb and PML enhanced the interaction between the two proteins. These results suggest that SUMO-1 functions to tether proteins to PML-containing nuclear bodies through post-translational modification and noncovalent protein-protein interaction.