Membrane permeability differentiation at the lipid divide
One of the deepest branches in the tree of life separates the Archaea from the Bacteria. These prokaryotic groups have distinct cellular systems including fundamentally different phospholipid membrane bilayers. This dichotomy has been termed the lipid divide and is assumed to bestow different biophysical and biochemical characteristics on each cell type. Classic experiments suggest that bacterial membranes are more permeable, yet systematic analysis based on direct measurements is absent. Here we develop a new approach for assessing the membrane permeability of cell-sized unilamellar vesicles, consisting of an aqueous medium enclosed by a single lipid bilayer. Comparing the permeability of twenty metabolites demonstrates that archaeal-type membranes are permeable to a range of compounds useful for core metabolic networks, including amino acids, sugars, and nucleobases. Surprisingly, permeability is much lower in bacterial-type membranes, in contradiction to current orthodoxy. We then show that archaeal permeability traits are specifically linked to both the methyl branches present on the archaeal phospholipid tails and the ether link between the tails and the head group. To explore this result further, we compare the abundance of transporter-encoding families present on genomes sampled from across the prokaryotic tree of life. Interestingly, archaea have a reduced repertoire of transporter gene families, consistent with an increased dependency on membrane permeation for a subset of metabolites. Taken together, these results demonstrate that the lipid divide demarcates a clear difference in permeability function with implications for understanding some of the earliest transitions in cell and protocell evolution.