BRcat domain found in early mitotic inhibitor (EMI) subfamily of F-box proteins
The EMI subfamily includes FBXO5 (EMI1) and FBXO43 (EMI2), which are anaphase-promoting-complex/cyclosome (APC/C) inhibitors that bind APC/C-CCD20 (Cell division cycle protein 20) and/or APC/C-CDH1 (CDC20 homolog 1) complexes. 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 the 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. FBXO43, also called FBX43, or endogenous meiotic inhibitor 2 (EMI2), plays a key role during the meiotic cell cycle. It is required to establish and maintain the arrest of oocytes at the second meiotic metaphase until fertilization. It probably acts by inhibiting the APC/C ubiquitin ligase. It may recognize and bind to some phosphorylated proteins and promote their ubiquitination and degradation. Both FBXO5 and FBXO43 contain 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 the EMI subfamily 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.",