Mediates E2-dependent protein ubiquitination. Acts as a negative regulator of abscisic acid signaling. Required for ABI5 degradation, by mediating its ubiquitination. Together with EDR1, may regulate endocytic trafficking and/or the formation of signaling complexes on trans-Golgi network (TGN)/ early endosome (EE) vesicles during stress responses. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with ABI5 and EDR1. A number of isoforms are produced. According to EST sequences. Expressed in all tissues of young seedlings. In flowering plants, only detected in the youngest part of the stem, anthers and the receptacle of immature siliques. Not found in mature leave, older parts of the stem, flower parts other than anthers or mature siliques. Expressed mainly in the actively growing and dividing cells. The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. Autophosphotylated and autoubiquitinated in vitro. Phosphorylation enhances self-ubiquitination. Autoubiquitinated in response to abscisic acid (ABA) and subsequently targeted to proteolysis. Plants are seedling lethal and are hypersensitive to glucose and abscisic acid. High accumulation of ABI5. The protein kinase domain is predicted to be catalytically inactive but PubMed:17194765 shows an in vitro activity.