Might act as an E3 ubiquitin-protein ligase, or as part of E3 complex, which accepts ubiquitin from specific E2 ubiquitin-conjugating enzymes and then transfers it to substrates (PubMed:12446796). Regulates negatively male gametophyte formation and double fertilization (PubMed:20478994). [E2 ubiquitin-conjugating enzyme]-S-ubiquitinyl-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-N(6)-ubiquitinyl-L-lysine. Binds 4 Zn(2+) ions per subunit. Protein modification; protein ubiquitination. Mostly expressed in closed flowers and, to a lower extent, in pollen. During male gametophyte development, strongly expressed in microspores and bicellular pollen, but decrease gradually during pollen maturation in tricellular pollen to finally disappear in mature pollen. Members of the RBR family are atypical E3 ligases. They interact with the E2 conjugating enzyme UBE2L3 and function like HECT-type E3 enzymes: they bind E2s via the first RING-type zinc finger, but require an obligate trans-thiolation step during the ubiquitin transfer, requiring a conserved active site Cys residue in the second RING-type zinc finger. The active site probably forms a thioester intermediate with ubiquitin taken from the active-site cysteine of the E2 before ultimately transferring it to a Lys residue on the substrate. Transcripts of KPL and ARI14, encoded by adjacent genes organized in a reverse orientation, have the potential to generate natural cis-antisense siRNAs (cis-nat-siRNAs) pair targeting ARI14 in sperm, thus leading to opposite expression levels during male gametophyte development. Belongs to the RBR family. Ariadne subfamily. Lacks four Cys residues in the RING-type zinc finger domain 1 and two Cys residues in the RING-type zinc finger domain 2 that are conserved features of the family.