phb-2

Gap: The molecular mechanism of the mitochondrial prohibitin complex is unresolved. It is not established whether the complex acts primarily as a membrane-bound chaperone/holdase for newly synthesized mitochondrial-encoded proteins, as a scaffold that organizes inner-membrane proteins within a defined (cardiolipin/phospholipid) microdomain, or as a regulator of inner-membrane proteostasis (e.g. of the m-AAA protease); nor is the direct molecular activity of phb-2 as a ring subunit expressible as a specific GO molecular function.

OPEN BIOLOGYONTOLOGY MF_DARK

What is known: It is firmly established that phb-1 and phb-2 bind each other and assemble into a ring-like, high-molecular-weight complex in the mitochondrial inner membrane, that the two subunits are interdependent (loss of one abolishes the complex), and that depletion is embryonic-lethal, disrupts gonad/germline differentiation, and alters mitochondrial biogenesis. What is NOT established is the biochemical mechanism by which the complex produces these effects.

Significance: Prohibitins are ubiquitous and essential across eukaryotes; resolving whether the complex is fundamentally a chaperone, a membrane scaffold, or a lipid/proteostasis organizer would explain a large body of pleiotropic phenotypes and the conserved link between mitochondrial membrane organization and ageing. The absence of a GO molecular-function term for a structural ring subunit is why the gene reads as MF-dark despite rich process/localization annotation.

What would resolve it: In vitro reconstitution / structural work on the worm (or conserved) PHB ring to test holdase vs scaffold activity; lipidomic and proximity-labeling mapping of the inner-membrane microdomain the complex organizes; separation-of-function alleles that uncouple candidate activities. In parallel, an ontology term for the structural/scaffolding molecular activity of a prohibitin-type ring subunit.

Provenance (the field's own admissions):

• "the true function of the mitochondrial prohibitin complex remains elusive" — PMID:26092086
• "as scaffold proteins that recruit membrane proteins to a specific lipid environment" — PMID:26092086

Proposed term (ontology gap):

• prohibitin complex structural constituent activity — phb-1/phb-2 and their orthologs have no adequate GO molecular-function term for their role as structural ring subunits and inner-membrane scaffolds. They are currently annotatable only at the complex/process level or with the generic 'structural molecule activity', leaving the gene MF-dark despite a well-defined cellular role.

Gap: How phb-2's mitophagy-receptor activity is molecularly integrated with, and separated from, its obligate structural role in the same protein is not resolved in C. elegans. It is unknown whether the worm phb-2 uses a defined LC3/LGG-family LIR motif in vivo (as shown for the mammalian ortholog), how outer-membrane rupture exposes the inner-membrane receptor to the cytosol in the physiological setting of paternal-mitochondria clearance, and which autophagy pathway (Parkin-independent) acts with phb-2 in the worm.

OPEN BIOLOGY RESIDUAL_SUBGAP

What is known: It is established that phb-2 is required for autophagic clearance of paternal mitochondria in worm (paternal phb-2 RNAi phenocopies atg-7 loss), that the mammalian ortholog binds LC3-II directly through a LIR while PHB1 binds only indirectly, and that a LIR point mutation abolishes mitophagy without disturbing the other prohibitin-dependent mitochondrial functions (i.e. the receptor and structural activities are separable in mammalian cells). What is NOT established is the in-vivo worm mechanism, its LGG-1/LGG-2 partner usage, and how the two activities of a single obligate subunit are coordinated.

Significance: PHB2 is the founding inner-mitochondrial-membrane mitophagy receptor, and paternal-mitochondria elimination in C. elegans is a premier in-vivo model of developmental mitophagy and uniparental mtDNA inheritance. Dissecting how one obligate structural subunit doubles as a receptor would clarify a conserved, Parkin-independent branch of mitophagy relevant to neurodegeneration and ageing.

What would resolve it: Map and mutate the worm phb-2 LIR (LGG-1/LGG-2 binding) with separation-of- function alleles; live-imaging of OMM rupture and phb-2 exposure during paternal mitophagy; epistasis with the worm autophagy machinery to place phb-2 in the Parkin-independent receptor pathway.

Provenance (the field's own admissions):

• "PHB2 interacts directly with LC3-II, whereas PHB1 interacts indirectly with LC3-II as a result of being part of a heterodimeric complex with PHB2" — PMID:28017329
• "the PHB2 LIR mutation does not affect previously described mitochondrial processes controlled by prohibitins, but does abrogate the ability of PHB2 to mediate mitophagy" — PMID:28017329

Gap: The mechanistic basis by which the SAME reduction of the prohibitin complex produces OPPOSITE ageing outcomes is unknown - prohibitin deficiency shortens the lifespan of otherwise wild-type animals yet extends the lifespan of diapause (daf-2), dietary-restricted, and respiration/fat-metabolism-compromised animals. The metabolic node at which the complex converts genetic/nutritional context into opposite longevity responses is undefined.

OPEN BIOLOGY RESIDUAL_SUBGAP

What is known: The context-dependence itself is well documented (life-shortening in wild type vs life-extending in daf-2/dietary-restricted/mitochondrial mutants), and depletion is known to change ATP levels, fat content, mitochondrial proliferation, and the whole-animal metabolome. The upstream signalling (insulin/IGF) and downstream metabolic readouts are mapped, but the causal molecular link through the complex is not.

Significance: This paradox is a clean, conserved example of how mitochondrial membrane organization gates lifespan in a metabolic-state-dependent manner; resolving it would connect prohibitin biology to insulin/IGF and dietary-restriction ageing pathways mechanistically.

What would resolve it: Epistasis and metabolic-flux analysis across the opposing backgrounds; identify the prohibitin-dependent metabolic step whose perturbation flips the longevity sign; test candidate mediators (fat mobilization, respiratory-chain assembly).

Provenance (the field's own admissions):

• "prohibitin deficiency shortens the lifespan of otherwise wild-type animals" — PMID:19812672
• "knockdown of prohibitin promotes longevity in diapause mutants or under conditions of dietary restriction" — PMID:19812672