  Lipid nanotube-vesicle networks are important channels for intercellular
communication and transport of matter. Experimentally observed in neighboring
mammalian cells, but also reproduced in model membrane systems, a broad
consensus exists on their formation and stability. Lipid membranes must be
composed of at least two molecular components, each stabilizing low (generally
a phospholipid) and high curvatures. Strong anisotropy or enhanced conical
shape of the second amphiphile is crucial for the formation of nanotunnels.
Anisotropic driving forces generally favor nanotube protrusions from vesicles.
In the present work, we report the unique case of topologically-connected
nanotubes-vesicles obtained in the absence of directional forces, in
single-molecule membranes, composed of an anisotropic bolaform glucolipid,
above its melting temperature, Tm. Cryo-TEM and fluorescence confocal
microscopy show the interconnection between vesicles and nanotubes in a
single-phase region, between 60{\textdegree} and 90{\textdegree}C under diluted
conditions. Solid-state NMR demonstrates that the glucolipid can assume two
distinct configurations, head-head and head-tail. These arrangements, seemingly
of comparable energy above the Tm, could explain the existence and stability of
the topologically-connected vesicles and nanotubes, which are generally not
observed for classical single-molecule phospholipid-based membranes above their
Tm.
