CNNM1

Model summary

A human multi-pass membrane transporter that assembles into higher-order complexes and uses an extracellular immunoglobulin-like module, a transmembrane ion-conducting core, and a cytosolic CBS/CNBH regulatory tail to control cellular magnesium balance. By coupling transport to intracellular nucleotide and ion sensing, it modulates Mg2+ homeostasis and organizes membrane-associated assemblies that fine-tune ion handling and trafficking.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture and its order along the sequence.

   The N-terminus carries IPR057492 (Metal transporter CNNM1/2/4, immunoglobulin-like domain, residues 41–208), an Ig-like fold that typically mediates extracellular or luminal adhesion and protein–protein recognition, thereby priming the polypeptide for regulated assembly and trafficking. Immediately downstream lies IPR002550 (CNNM, transmembrane domain, residues 218–414), a multi-pass membrane core that forms the ion-translocation pathway characteristic of CNNM transporters. The C-terminal half is dominated by cytosolic regulatory modules: IPR046342 (CBS domain superfamily, residues 412–568) and IPR044751 (Ion transporter-like, CBS domain, residues 428–553) together with IPR000644 (CBS domain, residues 502–568) define a tandem CBS “BNH/CBS” regulatory cassette that senses adenylate and divalent cations to allosterically tune transport. This cassette is followed by IPR061271 (Metal transporter CNNM-like, C-terminal CNBH domain, residues 576–796), a cytosolic CBS-like module that completes the ancient conserved domain protein scaffold captured by IPR045095 (Ancient conserved domain protein family, residues 156–813). The ordered layout—extracellular Ig-like module → multi-pass transmembrane core → intracellular CBS/CNBH regulatory tail—creates a membrane transporter whose activity is gated by cytosolic nucleotide/ion sensing.

2. 02

   This architecture causes a molecular function centered on regulated ion handling rather than soluble catalysis.

   The multi-pass CNNM core provides the conduit, while the CBS/CNBH tail confers cation-responsive control; together they support ion translocation and assembly into higher-order complexes. The prominent Ig-like and CBS modules also enforce multivalent contacts with cytosolic partners, which substantiates GO:0005515 protein binding as a defining activity of this transporter-scaffold.

3. 03

   From this molecular function, the biological process follows directly.

   CNNM-family transporters govern intracellular Mg2+ partitioning; the CBS/CNBH regulatory cassette is well-suited to couple transport to cellular energy and Mg2+ status. Consequently, the protein drives GO:0010960 magnesium ion homeostasis by modulating Mg2+ fluxes that set cytosolic and organellar pools.

4. 04

   The cellular context is dictated by the transmembrane core and the extracellular Ig-like domain.

   The presence of a multi-pass region and a luminal/extracellular Ig-like module places the protein in membranes as a multi-pass component, consistent with a membrane-resident transporter. The need for assembly and regulation via cytosolic CBS/CNBH domains further implies participation in macromolecular assemblies, aligning with GO:0032991 protein-containing complex. The membrane-embedded nature and trafficking signals support assignment to GO:0016021 integral component of membrane.

5. 05

   Mechanistically, I hypothesize that the Ig-like N-terminus organizes trafficking and surface complex formation, the CNNM transmembrane bundle forms the ion pathway, and the CBS/CNBH tail senses adenylates and Mg2+ to allosterically gate transport.

   This transporter likely nucleates complexes with cytosolic regulators and membrane partners to tune Mg2+ flux. Candidate interaction partners include other CNNM paralogs (forming hetero-oligomeric assemblies), phosphoinositide kinases/phosphatases that set membrane identity, and cytosolic Mg2+-sensing adaptors that dock onto the CBS/CNBH cassette. Such assemblies would coordinate membrane residency with Mg2+ homeostatic signaling.