Unique RING finger structure from the human HRD1 protein.

Artificial RING fingers (ARFs) are created by transplanting active sites of RING fingers onto cross-brace structures. Human hydroxymethylglutaryl-coenzyme A reductase degradation protein 1 (HRD1) is involved in the degradation of the endoplasmic reticulum (ER) proteins. HRD1 possesses the RING finger domain (HRD1_RING) that functions as a ubiquitin-ligating (E3) enzyme. Herein, we determined the solution structure of HRD1_RING using nuclear magnetic resonance (NMR). Moreover, using a metallochromic indicator, we determined the stoichiometry of zinc ions spectrophotometrically and found that HRD1_RING binds to two zinc atoms. The Simple Modular Architecture Research Tool database predicted the structure of HRD1_RING as a typical RING finger. However, it was found that the actual structure of HRD1_RING adopts an atypical RING-H2 type RING fold. This structural analysis unveiled the position and range of the active site of HRD1_RING that contribute to its specific ubiquitin-conjugating enzyme (E2)-binding capability.