Atypical Evolution of the F1Fo Adenosine Triphosphate Synthase Regulatory ATP6 subunit in Glacier Ice Worms (Annelida: Clitellata: Mesenchytraeus)

A histidine-rich extension of the mitochondrial F0 subunit ATP6 from the ice worm Mesenchytraeus solifugus increases ATP synthase activity in bacteria

Bioenergetic profiles of psychrophiles across domains of life are unusual in that intracellular ATP levels increase with declining temperature. Whole-transcriptome sequencing of the glacier ice worm Mesenchytraeus solifugus revealed a unique C-terminal extension on the ATP6 protein, which forms part of the proton pore of mitochondrial ATP synthase (Complex V). This extension, positioned near the proton exit pore, comprises alternating histidine residues thought to increase proton flux through Complex V leading to elevated ATP synthesis. To test this hypothesis, we fused the M. solifugus C-terminal extension to Escherichia coli AtpB (the ATP6 orthologue) and observed a ~ 5-fold increase in ATP synthesis. This enhancement was unidirectional as we observed no change to ATP hydrolysis rates. These findings offer an avenue for identifying critical factors associated with ice worm adaptation.

The Glacier Ice Worm, Mesenchytraeus solifugus, Elevates Mitochondrial Inorganic Polyphosphate (PolyP) Levels in Response to Stress

The inorganic polymer, polyphosphate (polyP), is present in all organisms examined to date with putative functions ranging from the maintenance of bioenergetics to stress resilience and protein homeostasis. Bioenergetics in the glacier-obligate, segmented worm, Mesenchytraeus solifugus, is characterized by a paradoxical increase in intracellular ATP levels as temperatures decline. We show here that steady-state, mitochondrial polyP levels vary among species of Annelida, but were elevated only in M. solifugus in response to thermal stress. In contrast, polyP levels decreased with temperature in the mesophilic worm, Enchytraeus crypticus. These results identify fundamentally different bioenergetic strategies between closely related annelid worms, and suggest that I worm mitochondria maintain ATP and polyP in a dynamic equilibrium.

Structural Evolution of the Glacier Ice Worm Fo ATP Synthase Complex

The segmented annelid worm, Mesenchytraeus solifugus, is a permanent resident of temperate, maritime glaciers in the Pacific northwestern region of North America, displaying atypically high intracellular ATP levels which have been linked to its unusual ability to thrive in hydrated glacier ice. We have shown previously that ice worms contain a highly basic, carboxy terminal extension on their ATP6 regulatory subunit, likely acquired by horizontal gene transfer from a microbial dietary source. Here we examine the full complement of F₁F₀ ATP synthase structural subunits with attention to non-conservative, ice worm-specific structural modifications. Our genomics analyses and molecular models identify putative proton shuttling domains on either side of the F₀ hemichannel, which predictably function to enhance proton flow across the mitochondrial membrane. Other components of the ice worm ATP synthase complex have remained largely unchanged in the context of Metazoan evolution.