(A) The interface between the Rcat (pink ribbon) and RING0 (wheat surface) domains for parkin are shown highlighting important residues near the catalytic site. The three residues (Cys⁴³¹, His⁴³³ and Glu⁴⁴⁴) important for ubiquitin transfer are shown in addition to several residues found at the Rcat (Trp⁴⁶² and Phe⁴⁶³), RING0 (Lys¹⁶², Trp¹⁸³, Pro¹⁸⁰ and Val¹⁸⁶) interface (PDB code 4I1F [35]). (B) A portion of the interface between the N-lobe (grey surface) and the catalytic region of the C-lobe (green ribbon) in NEDD4 is shown (PDB code 4BBN [122]). The catalytic cysteine (Cys⁸⁶⁷) resides between two β-strands similar to the position in parkin and HHARI. Two other residues important for catalysis (His⁸⁶⁵ and Asp⁹⁰⁰) are arranged in a mirror fashion compared to the Rcat domain in parkin and HHARI although Asp⁹⁰⁰ is not visible in the X-ray structure. In both structures the two β-strands were superimposed to achieve similar protein orientations.