Structure-Function Relationships in Pure Archaeal Bipolar Tetraether Lipids

Abstract

Archaea, the third fundamental domain of life are distinguished from bacteria and eukaryotes due to the presence of unique lipids in their cell membranes. Archaeal bipolar lipids are among the most unusual lipids occurring in nature because of their presumed ability to span the entire membrane. They are challenging to extract in pure form from natural sources or synthesize chemically, and as a result, prior studies on pure lipids have been limited. Here we have utilized synthesis to enable in-depth biophysical investigations on a series of archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids having symmetric or unsymmetric combinations of polar head groups. We showed that these lipids self-assemble to form membrane-bound vesicles in aqueous media, encapsulate polar molecules, and reconstitute a functional integral membrane protein. Membrane thicknesses and electron density profiles were investigated by performing small-angle X-ray scattering (SAXS) studies and cryogenic electron microscopy (cryo-EM) imaging on unilamellar vesicles of GDGT lipids. SAXS studies on bulk aqueous dispersions of GDGT lipids over a large temperature range (10-90 °C) allowed us to identify lamellar and non-lamellar phases and their interconversions under various buffer conditions. We also studied how the propensity to form various mesophases is reflected in the functional behavior of the GDGT membranes. Specifically, we asked whether vesicles overwhelmingly composed of GDGTs can undergo fusion as it is difficult to conceptualize such behavior with the assumption that such membranes have a monolayer structure. Interestingly, we observed that GDGT vesicles undergo fusion with influenza virus with lipid mixing kinetics comparable to that with vesicles composed of typical monopolar phospholipids. Our results suggest that GDGT membranes may consist of regions with a bilayer structure which facilitates fusion and thus offer insight into how archaea may perform important physiological functions that require dynamical membrane behavior.