F-box and leucine-rich repeat protein 5 (FBXL5): sensing intracellular iron and oxygen

Abstract

Though essential for many vital biological processes, excess iron results in the formation of damaging reactive oxygen species (ROS). Therefore, iron metabolism must be tightly regulated. F-box and leucine-rich repeat protein 5 (FBXL5), an E3 ubiquitin ligase subunit, regulates cellular and systemic iron homeostasis by facilitating iron regulatory protein 2 (IRP2) degradation. FBXL5 possesses an N-terminal hemerythrin (Hr)-like domain that mediates its own differential stability by switching between two different conformations to communicate cellular iron availability. In addition, the FBXL5-Hr domain also senses O2 availability, albeit by a distinct mechanism. Mice lacking FBXL5 fail to sense intracellular iron levels and die in utero due to iron overload and exposure to damaging levels of oxidative stress. By closely monitoring intracellular levels of iron and oxygen, FBLX5 prevents the formation of conditions that favor ROS formation. These findings suggest that FBXL5 is essential for the maintenance of iron homeostasis and is a key sensor of bioavailable iron. Here, we describe the iron and oxygen sensing mechanisms of the FBXL5 Hr-like domain and its role in mediating ROS biology.

Keywords: FBXL5; Hemerythrin; Iron; Iron regulatory proteins; Oxygen; Reactive oxygen species.

Figure 1. Cellular iron homeostasis is regulated by Iron Regulatory Proteins (IRPs)

In iron-depleted cells, IRP binding to an IRE in the 5’ UTR (e.g. ferritin) inhibits translation initiation while IRP binding to IREs in the Transferrin receptor 1 (Tfr1) 3’ UTR promotes mRNA stabilization (left). Under high iron conditions, IRPs lose their IRE binding capacity either through Fe-S cluster assembly within IRP1 or proteasomal degradation of IRP2 (right).

Figure 2. Model for the regulation of mammalian cellular iron homeostasis by FBXL5

A, Under iron and oxygen replete conditions, the FBXL5-Hr domain resides in a folded compact structure promoting FBXL5 accumulation, assembly into a E3 ubiquitin ligase complex (SCF), and subsequent IRP2 degradation to promote iron storage and inhibit iron uptake. B, Limiting iron and oxygen levels, and possibly ROS, induce FBXL5 degradation. Low iron availability hampers assembly of the di-iron center. Consequently, the Hr domain unfolds to expose a degron located within this domain which is recognized by an unidentified E3 ubiquitin ligase. FBXL5 is subsequently degraded by the proteasome. As a result, IRP2 is stabilized favoring increased iron uptake and attenuating iron storage.

Figure 3. Schematic representation of the di-iron center from deoxy-Hr and FBXL5-Hr domain

A, In canonical Hr domains the Fe atoms are bridged by an aspartate and a glutamate residue. Five histidines complete the coordination sphere of the Fe atoms. B, Among the unique features of the FBXL5-Hr is a glutamate (red, upper left side) that replaces a coordinating histidine. In addition, the iron atoms are bridged by two glutamate residues (blue, near center middle and lower parts). Adapted with permission from Biochim Biophys Acta. 2012 Sep;1823(9):1484-90 (license # 3300850211726).

Figure 4. Crystal structure of FBXL5 Hr domain

A, Ribbon representation of the Hr domain of human FBXL5 (PDB code 3V5Z). Helices are shown in blue and the atypical fifth C-terminal helix (far left, near vertical helix) is shown in purple. B, Amino acids constituting the first (H57, E61, E58, H80, H15, E130 and H126) and second (H158 and N62) coordination shell of the di-iron center (red) are shown. The blue sphere between the di-iron center (Fe1 and Fe2) is a μ-oxygen species.