SLC25A52

Model summary

A multi-pass inner-membrane transporter in human mitochondria that selectively imports NAD+ into the matrix. By cycling a six-helix carrier core through alternating-access states, it moves adenine nucleotide derivatives across the inner membrane to sustain mitochondrial dehydrogenase and sirtuin pathways. Its activity supplies matrix NAD+ pools, thereby coupling nucleotide flux to energy metabolism and redox balance within the organelle.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide is dominated by IPR023395 (Mitochondrial carrier protein domain superfamily, residues 30–297), the hallmark scaffold of the mitochondrial carrier family that forms a tripartite, six-helix bundle in the inner membrane. This scaffold is reinforced by three tandem transmembrane repeats annotated as IPR018108 (Mitochondrial carrier protein, transmembrane region, residues 28–111, 116–202, and 209–296). The ordered arrangement of three repeats creates a pseudo-symmetrical pore with matrix- and intermembrane-space gates that alternately expose a central binding site. Superimposed on this generic carrier framework, IPR052465 (Mitochondrial NAD+ Carrier family, residues 3–296) specifies substrate class: a dedicated NAD+-transporting branch of the carrier superfamily. The combination of a carrier superfamily core with a NAD+-carrier family signature causally dictates selective transmembrane transport of adenine dinucleotide derivatives.

2. 02

   This carrier architecture enforces a molecular function of transmembrane transporter activity.

   The six-helix bundle and alternating-access gating produce vectorial movement of small, charged metabolites; in this case, the NAD+-carrier family identity directs the substrate to NAD+, aligning with GO:0000295 adenine nucleotide transmembrane transporter activity. The mechanistic basis is a solute-binding cavity whose conformational cycling is driven by the membrane potential and salt-bridge networks unique to mitochondrial carriers.

3. 03

   Transport of NAD+ across the inner membrane directly establishes mitochondrial nucleotide transmembrane transport (GO:1990542) and, more specifically, purine nucleotide transmembrane transport (GO:0072530).

   By importing NAD+ into the matrix, the transporter sustains matrix dehydrogenase reactions and sirtuin-dependent pathways, thereby coupling nucleotide flux to central energy metabolism.

4. 04

   The carrier superfamily and the three transmembrane repeats inherently place the protein in a membrane environment.

   The presence of the mitochondrial carrier fold and the NAD+-carrier family signature together localize the protein to mitochondria (GO:0005739), and the six-helix bundle and multi-pass topology specify an integral membrane residency consistent with a multi-pass inner-membrane transporter.

5. 05

   From this mechanistic base, I hypothesize that the protein operates as a gated NAD+ importer that alternates exposure of its binding site between the cytosolic side of the inner membrane and the matrix, using the canonical carrier salt-bridge network to switch between states.

   Likely interaction partners include other inner-membrane carrier assemblies and their assembly pathway, which could transiently coordinate carrier biogenesis and stability. Functionally, the transporter is poised to interface with matrix NAD-dependent enzymes and regulators; thus, it may form transient complexes with inner-membrane assembly factors and indirectly align with matrix pathways that consume NAD+.