PROSITE documentation PDOC52084

NCOA4 HERC2-binding domain (HBD) profile

Iron is essential for the survival of nearly all organisms as it serves as a cofactor for a host of biochemical processes including oxygen storage, oxidative phosphorylation, and enzymatic reactions required for cellular proliferation. However, the levels of free iron in a cell must be tightly controlled to avoid the generation of reactive oxygen species (ROS) via the Fenton reaction. As such, iron metabolism is a tightly regulated process controlled by a network of iron-dependent proteins. The cell has evolved mechanisms whereby iron can be sequestered and released from protein complexes in response to changing iron levels. One such protein is ferritin, which forms a complex of 24 subunits (consisting of a mixture of ferritin heavy and light chains, FTH1 and FTL, respectively) capable of storing up to 4500 iron atoms. Under iron-depleted conditions, iron can be released through the selective autophagy of ferritin (ferritinophagy), a process mediated by the autophagy receptor NCOA4. To prevent potential deleterious effects induced by excess cellular iron, the cellular protein level of NCOA4 is regulated by the E3 ubiquitin ligase HERC2 that can specifically interact with NCOA4 and induce its proteasomal degradation under iron-replete conditions. Under iron-replete conditions, the iron loading of NCOA4 can facilitate the interaction of NCOA4 with the E3 ligase HERC2, thereby promoting the subsequent HERC2-mediated ubiquitination and proteasomal degradation of NCOA4 to allow ferritin accumulation for storing cellular excess iron ions. By contrast, under iron-depleted conditions, NCOA4 is ineffectively recognized and ubiquitinated by the E3 ligase HERC2, resulting in the augmentation of the cellular protein level of NCOA4 for targeting iron-laden ferritin for autophagic degradation to supply free iron [1,2].

NCOA4 mainly contains an N-terminal coiled-coil domain that mediates the self-polymerization of NCOA4, a HECT and RLD-containing E3 ubiquitin protein ligase 2 (HERC2)-binding domain (HBD), and a C-terminal ferritin-binding motif (FBM). The HBD of NCOA4 senses the cellular iron level and is responsible for binding to HERC2 in an iron-sulfur cluster-dependent manner. The HBD of NCOA4 can specifically coordinate a [2Fe-2S] cluster, and can exist in two different states, the apo state and the [2Fe-2S] cluster-bound state. The [2Fe-2S] cluster-bound form but not the apo form of NCOA4 HBD can be effectively recognized by HERC2 through the cooperation of the CPH and INBD domains within HERC2, during which the INBD of HERC2 plays a major role in recognizing the [2Fe-2S] cluster-bound NCOA4 HBD [2].

The NCOA4 HBD domain adopts an extended configuration containing an N-terminal short α-helix and a C-terminal elongated α-helix that mediates the dimer formation of NCOA4 (see <PDB:9L93>). The loop region connecting the two α-helices of each NCOA4 HBD domain contains the [2Fe-2S] cluster, which is coordinated by four Cys residues [2].

The profile we developed covers the entire NCOA4 HBD domain.