exbB

Gene Description

ExbB is an integral inner (cytoplasmic) membrane, multi-pass protein that forms part of the TonB-ExbB-ExbD energy-transducing complex in Gram-negative bacteria. Together with ExbD, ExbB assembles into a membrane-embedded oligomeric motor (an ExbB pentamer enclosing an ExbD dimer is the consensus architecture) that harnesses the proton motive force of the inner membrane to energize TonB. Energized TonB physically contacts the TonB box of TonB-dependent outer-membrane transporters (TBDTs), driving conformational changes that open these gated beta-barrel receptors for active import of scarce, receptor-bound substrates such as ferric-siderophore complexes, heme, and vitamin B12 into the periplasm. ExbB itself is not a substrate-specific transporter; rather it is the proton-conducting, energy-coupling stator subunit that powers many different TBDTs, and it stabilizes TonB and protects ExbD from proteolysis. The protein belongs to the ExbB/TolQ family and shares a MotA/TolQ/ExbB proton-channel architecture with the flagellar stator MotA. In Pseudomonas putida KT2440 it is encoded at PP_5306 in the exbB-exbD-tonB (PP_5306-PP_5308) operon, where the system is required for iron-dependent growth and TonB-dependent nutrient acquisition.

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005886 plasma membrane	IEA GO_REF:0000120	ACCEPT	Summary: ExbB is an integral inner (cytoplasmic/plasma) membrane multi-pass protein, consistent with UniProt subcellular location and the conserved biology of the ExbB/TolQ family. This is a correct, core localization for the gene product.
GO:0016020 membrane	IEA GO_REF:0000120	KEEP AS NON CORE	Summary: Correct but generic; this is a less informative parent of the more specific plasma (inner) membrane localization that is also annotated. Retained as a true but non-core, redundant statement.
GO:0017038 protein import	IEA GO_REF:0000118	MARK AS OVER ANNOTATED	Summary: This TreeGrafter-propagated term mischaracterizes the core function of ExbB. The defining role of ExbB is to couple the proton motive force to energize TonB, powering TonB-dependent outer-membrane transport of small nutrients (ferric-siderophores, heme, vitamin B12), not the import of proteins. The TonB system can in some organisms energize uptake of certain proteinaceous substrates (e.g., group B colicins and some toxins), so the term is not strictly false, but as applied here it is an over-annotation that does not represent the gene's principal biological role and conflates a niche activity with the main function. Reason: The 'protein import' definition (targeting/directed movement of proteins into a cell or organelle) does not capture ExbB's PMF-coupled energization of small-nutrient uptake; this is an electronic over-propagation.
GO:0022857 transmembrane transporter activity	IEA GO_REF:0000002	MODIFY	Summary: ExbB does not itself transport the receptor-bound substrates (iron siderophores, heme, B12); those are imported by the outer-membrane TBDTs. ExbB is the proton-conducting, energy-coupling stator subunit of the ExbB-ExbD-TonB motor. A generic 'transmembrane transporter activity' is misleading; the specific, mechanistically accurate molecular function is proton transmembrane transporter activity (the MotA/TolQ/ExbB proton channel conducts protons across the inner membrane to convert PMF into mechanical work on TonB). Reason: ExbB is a PMF-coupled energizer/proton channel, not a substrate-specific transmembrane transporter; the InterPro-derived generic term should be replaced with the proton-channel molecular function. Proposed replacements: proton transmembrane transporter activity
GO:0055085 transmembrane transport	IEA GO_REF:0000002	MODIFY	Summary: Broadly true but uninformatively generic. The directly catalyzed process for ExbB is proton transmembrane transport across the inner membrane, which generates the energy used to drive TonB-dependent outer-membrane uptake. Replacing the generic term with proton transmembrane transport better reflects the mechanism while remaining well supported by the conserved family biology. Reason: The generic transmembrane transport term should be specialized to the proton movement that ExbB actually mediates. Proposed replacements: proton transmembrane transport