SUMOylation proceeds by an enzyme cascade in which SUMO is sequentially processed by the E1 SAE1/SAE2, then E2 UBC9 before ligation to protein substrates with support from E3 ligases. We have not categorically established E3 ligase activity and ZBTB16 lacks the Homologous-to-E6AP-Carboxyl-Terminus (HECT), UFD2-homology (U-box) or Really-Interesting-New-Gene (RING) domains that are associated with E3 activity. However, other BTB-domain-containing proteins, for instance, the Kelch-like ECH-associated protein 1 that forms the Cullin 3 E3 ubiquitin ligase complex, function as substrate adaptor proteins. Our mapping experiments detected that ZBTB16 interacted with ASC, UBC9 and/or SUMO1 via its BTB/POZ and RD2 domains. Accordingly, ZBTB16 may control SUMOylation by modifying noncovalent interactions within an as-yet unrecognised E3 enzyme complex. Computational (Joined Advanced SUMOylation Site and SIM analyser (JASSA v4) and GSP-SUMO 2.0) analysis predicts a SUMO-interaction motif (ILEIE at residue positions 108-112) on the BTB/POZ domain of ZBTB16 that accords with this activity. Otherwise, ZBTB16 may function more generally to regulate the sequestration of protein partners in order to advance this posttranslational modification.
