NIPAL1

Model summary

A human multi-pass membrane transporter that mediates high-affinity uptake of magnesium. Its architecture forms an oligomeric pathway that captures extracellular magnesium at the cell surface and recovers it within early endosomes, sustaining cellular magnesium balance. The transporter resides at the plasma membrane and cycles through early endosomal membranes, where assembly with trafficking and sorting factors positions and recycles it to maintain uptake capacity and endocytic flux.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The core of the protein is defined by IPR008521 (Magnesium transporter NIPA family, residues 66–375), which spans nearly the full length and establishes a dedicated Mg2+-transport scaffold. Embedded within this framework is IPR037185 (Multidrug transporter EmrE superfamily, residues 114–187). The EmrE-like superfamily signature indicates a compact, multi-pass membrane fold that typically forms oligomeric pores or alternating-access carriers. The ordered layout—an extensive NIPA-family envelope that houses an EmrE-like transmembrane core—causally dictates a multi-pass membrane transporter that coordinates divalent cations. The NIPA family is specialized for Mg2+ selectivity and high-affinity uptake, while the EmrE-like module supplies the structural mechanics for ion translocation across lipid bilayers.

2. 02

   From this architecture, the molecular function resolves into two layers.

   First, the EmrE-like transmembrane core and the NIPA scaffold together create a conduit for Mg2+ movement, which necessitates direct macromolecular contacts for assembly and regulation; this supports GO:0005515 protein binding as an intrinsic property of a transporter that often oligomerizes and recruits trafficking factors. Second, the physicochemical specialization of the NIPA family explains why the transporter mediates high-affinity Mg2+ uptake: the extended NIPA envelope tunes selectivity and gating, while the EmrE-like core executes the transmembrane pathway.

3. 03

   This molecular function naturally feeds into the biological process of GO:0015693 magnesium ion transport.

   High-affinity Mg2+ uptake at the cell surface and within endosomal systems maintains cytosolic Mg2+ pools that fuel ribosomal function, ATP-dependent metabolism, and enzyme cofactor supply. The transporter’s architecture causes net Mg2+ influx under physiological gradients, thereby sustaining cellular magnesium economy.

4. 04

   Cellular localization follows from both the fold and trafficking cues.

   The multi-pass arrangement and EmrE-like superfamily membership place the protein in membranes, consistent with a stable integral presence. The NIPA family’s role in uptake and recycling aligns with GO:0005886 plasma membrane for cell-surface import. Endosomal residency is explained by the need to salvage Mg2+ during cargo sorting; the architecture and binding propensity support retention and recycling at GO:0031901 early endosome membrane. The experimentally observed distribution between the plasma membrane and early endosomes coheres with a transporter that cycles between uptake sites and sorting hubs.

5. 05

   Mechanistically, I hypothesize that the transporter assembles as an oligomer whose EmrE-like core alternates access between cytosolic and luminal sides to drive Mg2+ uptake.

   At the plasma membrane, it captures extracellular Mg2+; at early endosomes, it recovers Mg2+ from endocytosed pools. Protein binding likely stabilizes higher-order complexes and couples to endocytic adaptors. Candidate interaction partners include clathrin-mediated endocytic machinery (e.g., AP-2 and clathrin heavy chain) and early endosome organizers (such as RAB5 and EEA1), which would position and recycle the transporter. Cytosolic Mg2+-sensing or buffering proteins (e.g., SLC8A family members or polyphosphate-binding modules) may transiently associate to tune flux and maintain Mg2+ homeostasis.