RING1 of RBRs maintains features characteristic of canonical RINGs. (a) Structures of RING domains are displayed with Zn2+ coordinating residues as yellow sticks and Zn2+ ions displayed as grey spheres. A conserved hydrophobic residue important for E2 binding is shown as orange sticks. The structures are (from left to right) the E3 ligase CNOT4 (blue) bound to the E2 UbcH5b (purple) (PDB ID 1UR6) (the E2 active site is shown as yellow spheres); the heterodimeric RING E3 ligase BRCA1 (blue)/BARD1 (green) (PDB ID 1JM7); TRAF6 (PDB ID 3HCT); RING1 of the RBR E3 RNF144 (PDB ID 1WIM). (b) Multiple sequence alignment of the RING domains of CNOT4, BRCA1, TRAF6, and RNF144. Coloring in the multiple sequence alignment corresponds with the colors in the structures, highlighting residues important for Zn2+ coordination and E2 binding. Sequences were aligned using CLUSTALW and manually adjusted based on structure .