Table 1.

E3-Ub ligases types

Main types of E3 ligases	Characteristics	Action mode
RING E3 ligases	Contains a RING domain or U-box domain	Ub is transferred in one step from the E2-conjugating enzyme to the substrate
Homologous to the HECT E3 ligases	Subfamilies: 1. RCC1-like domains; 2. tryptophan-tryptophan domains; 3. ligases lacking these domains	Ub is transferred in two steps: these ligases first transfer Ub from the E2-conjugating enzyme to themselves and subsequently to the substrate
RBR E3 ligases	Contain an IBR which separates two predicted RING domains (RING1 and RING2)

RCC1: regulator of chromosome condensation 1; RBR: RING-in-between-RING; HECT: E6AP carboxyl terminus; IBR: in between ring