BRcat domain found in F-box only protein 5 (FBXO5)
FBXO5, also called FBX5 or early mitotic inhibitor 1 (EMI1), acts as a regulator that inhibits the anaphase-promoting complex/cyclosome (APC/C), which controls cell cycle progression through the sequential degradation of various substrates from S phase to early mitosis. During mitotic cell cycle, it plays a role as both substrate and inhibitor of the APC-FZR1 complex. During G1 phase, it plays a role as substrate of the APC-FZR1 complex E3 ligase. FBXO5 contains an F-box domain, and the first half of the 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 changed to RING1-BRcat (benign-catalytic)-Rcat (required-for-catalysis) recently. The RBR domain uses an auto-inhibitory mechanism to modulate ubiquitination activity, as well as a hybrid mechanism that combines aspects from both RING and HECT E3 ligase functions to facilitate the ubiquitination reaction. This model corresponds to the BRcat domain of FBXO5 that adopts the same fold as the Rcat domain while lacking the catalytic cysteine residue and ubiquitination activity.
Jiyao W et al.(2023). "The conserved domain database in 2023.",