RING finger, HC subclass, found in two RING fingers and DRIL [double RING finger linked] 1 (TRIAD1), ankyrin repeat and IBR domain-containing protein 1 (ANKIB1) and similar proteins
TRIAD1, also known as ariadne-2 (ARI-2), protein ariadne-2 homolog, Ariadne RBR E3 ubiquitin protein ligase 2 (ARIH2), or UbcM4-interacting protein 48, is a RBR-type E3 ubiquitin-protein ligase that catalyzes the formation of polyubiquitin chains linked via lysine-48 as well as lysine-63 residues. Its auto-ubiquitylation can be catalyzed by the E2 conjugating enzyme UBCH7. TRIAD1 has been implicated in hematopoiesis, specifically in myelopoiesis, as well as in embryogenesis. ANKIB1 is a RBR-type E3 ubiquitin-protein ligase that may function as part of E3 complex, which accepts ubiquitin from specific E2 ubiquitin-conjugating enzymes and then transfers it to substrates. Both TRIAD1 and ANKIB1 contain a RBR domain that was previously known as RING-BetweenRING-RING domain or TRIAD [two RING fingers and a DRIL (double RING finger linked)] domain. Based on current understanding of the structural biology of RBR ligases, the nomenclature of RBR has been corrected as RING-BRcat (benign-catalytic)-Rcat (required-for-catalysis) recently. The RBR (RING1-BRcat-Rcat) domain use an auto-inhibitory mechanism to modulate ubiquitination activity, as well as a hybrid mechanism that combines aspects from both RING and HECT E3 ligase function to facilitate the ubiquitination reaction. This family corresponds to the RING domain, a C3HC4-type RING-HC finger required for RBR-mediated ubiquitination. In contrast to TRIAD1, ANKIB1 harbors an extra N-terminal ankyrin repeats domain.