FRRS1

Model summary

A multi-pass membrane redox enzyme that uses an extracellular Reeler module and a DOMON scaffold to position and regulate a cytochrome b561-like electron-transfer core. This architecture enables electron flow that reduces ferric to ferrous iron at membrane interfaces, thereby stabilizing cellular iron balance. The protein traffics through the secretory pathway, accumulates at the plasma membrane and intracellular membranes, and is frequently incorporated into extracellular vesicles. Zinc coordination by the extracellular modules likely stabilizes structure and modulates activity, while the transmembrane redox center executes metal-ion oxidation–reduction chemistry central to iron homeostasis.

Predicted Gene Ontology terms

1. 01

   I begin with the domain architecture.

   The N-terminus carries IPR002861 (Reeler domain, residues 13–179), embedded within IPR042307 (Reeler domain superfamily, residues 27–158). Reeler repeats are extracellular/lumen-facing modules that mediate protein–protein and protein–matrix interactions and often guide trafficking through the secretory pathway. Midway through the sequence sits IPR005018 (DOMON domain, residues 216–331), a β-sandwich module classically used for redox and ligand recognition in extracellular and membrane proteins. The C-terminal half transitions into IPR006593 (Cytochrome b561/ferric reductase transmembrane domain, residues 335–534), a multi-pass heme-bearing redox core that conducts electron transfer across membranes. The ordered layout—Reeler (extracellular) → DOMON (extracellular/lumenal) → cytochrome b561-like transmembrane core—creates a conduit that captures extracellular/lumenal cues and transmits electrons through a multi-pass membrane module.

2. 02

   This architecture dictates molecular function.

   The cytochrome b561/ferric reductase transmembrane domain provides the electron-transfer pathway that reduces Fe(III) to Fe(II). The DOMON and Reeler modules position and stabilize substrate-binding and assembly surfaces, while the b561 core executes metal-ion redox chemistry. Together, these features cause oxidoreductase activity acting on metal ions (GO:0016722). The presence of a metal-binding extracellular scaffold and a heme-based transmembrane redox center also rationalizes ferroxidase chemistry (GO:0004322): the same redox machinery can catalyze reversible Fe valence transitions, buffering iron speciation. The Reeler/DOMON surfaces frequently coordinate divalent metals; this supports zinc ion binding (GO:0008270), which can modulate local redox and structural properties.

3. 03

   From function to process, iron redox control is a direct lever of intracellular iron ion homeostasis (GO:0006879).

   By reducing Fe(III) at membrane interfaces and stabilizing iron handling, the protein tunes labile Fe pools and mitigates oxidative stress, thereby sustaining cellular iron economy.

4. 04

   Cellular location follows from the modules and trafficking logic.

   The Reeler and DOMON domains are characteristic of secretory-pathway and cell-surface proteins, steering the protein through the Golgi apparatus (GO:0005794) en route to membranes. The multi-pass cytochrome b561 core anchors the protein in membranes, consistent with a residence at the plasma membrane (GO:0005886) and other intracellular membranes. Packaging into extracellular vesicles explains detection in extracellular exosomes (GO:0070062), a common fate for membrane proteins traversing the secretory/endosomal continuum.

5. 05

   Mechanistically, I hypothesize that the N-terminal Reeler and DOMON modules recruit extracellular/lumenal partners and possibly bind Zn2+ as a structural or regulatory cofactor, while the C-terminal cytochrome b561-like core shuttles electrons to catalyze Fe(III) reduction at the membrane interface.

   This arrangement positions the enzyme to regulate iron redox balance at the cell surface and along the secretory pathway. Likely interaction partners include secretory-pathway chaperones and cargo receptors that guide Golgi-to-plasma membrane trafficking, as well as exosome biogenesis factors that sort the protein into vesicles. At the cell surface, transient complexes with iron-handling proteins and membrane scaffolds would localize and tune the redox activity that sustains intracellular iron homeostasis.