Quinone-transporting filaments expand bioenergetic capacity in Gram-positive Bacillota

All living cells need energy, and most generate it through respiration, a series of chemical reactions that is normally confined to the cell membrane. Because this machinery takes up space, a cell’s energy-generating capacity has long been thought to depend on how much membrane it can build. Complex cells solve this problem using mitochondria packed with folded membranes, while some bacteria enlarge or reshape their own membranes. In each case, producing more energy requires changing the cell’s internal architecture.

Our study reveals an entirely different, and previously unknown, solution. We found that some bacteria build long, hollow filaments that extend their respiratory machinery away from the membrane and into the cell. These filaments are made from proteins and lipids with a hollow interior that electron-carrying molecules can move through, effectively creating power cables within the cell. This allows many energy-processing enzymes to work beyond the cell membrane, expanding respiratory capacity without the bacterium having to build additional membranes or fundamentally reorganise its structure.

The discovery challenges the conventional view that respiration must remain confined to a membrane surface. Instead, bacteria can construct what is effectively a microscopic power grid: protein-based “extension cables” that carry part of the respiratory chain into the cell’s interior. Related systems occur across hundreds of bacterial species, suggesting that this is not a one-off curiosity but a widespread evolutionary strategy.

This new class of biological structure, part protein machine, part artificial membrane, expands our understanding of how cells can organise energy production. It also offers a striking example of evolution solving a fundamental physical constraint in an unexpected way, and could inspire synthetic molecular systems that transport and manage energy outside conventional membranes.