SUMOylation is a posttranslational modification that regulates various aspects of protein functions, from stability and activity to localization in cellular compartments. Through its regulatory role, SUMOylation contributes to various eukaryotic cellular processes including DNA repair, transcription and nuclear transport. Followed by the cascade of enzymatic reactions, E3 ligases have a key role in the fast and efficient handover of SUMO to the molecules of a substrate. Although almost one-third of proteins are subject to SUMOylation, so far there are only two well-characterized human E3 ligases. The final effect of SUMOylation is also unclear for most substrates. To improve the understanding of SUMOylation, the focus of this project was on biochemical and structural characterization of one SUMO E3 ligase, TOPORS, and a set of SUMOylation substrates from the ZBTB family. An E3 ligase, TOPORS, unique because it displays both ubiquitin and SUMO ligase activities, was chosen because it has been poorly explored so far and its structure and mechanism remain unknown. During the project, we made preliminary bioinformatic and biochemical observations about its middle region which encompasses a putative DNA-binding PWI domain and a fragment that has the SUMO E3 ligase activity. As transcription factors are among the most frequently targeted proteins for SUMOylation, the second part of the project investigated ZBTB8A as a representative member of the large family of human transcription factors. While the effect of SUMOylation on this protein has not been determined during the project, we performed a structural analysis of a crystallized fragment of ZBTB8A, revealing that it organizes into filament structures via its BTB domain.