Role of Squalene in the Organization of Monolayers
Derived from Lipid Extracts of Halobacterium salinarum
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Abstract
We have studied interfacial compressibility
and lateral organization
in monolayer configurations of total (squalene containing) and polar
(squalene-devoid) lipid extracts of Halobacterium salinarum NRC-1, an extremely halophilic archaeon. Pressure–area isotherms
derived from Langmuir experiments reveal that packing characteristics
and elastic compressibility are strongly influenced by the presence
of squalene in the total lipid extract. In conjunction with control
experiments using mixtures of DPhPC and squalene, our results establish
that the presence of squalene significantly extends elastic area compressibility
of total lipid extracts, suggesting it has a role in facilitating
tighter packing of archaeal lipid mixtures. Moreover, we find that
squalene also influences spatial organization in archaeal membranes.
Epifluorescence and atomic force microscopy characterization of Langmuir
monolayers transferred onto solid hydrophilic substrates reveal an
unusual domain morphology. Individual domains of microscopic dimensions
(as well as their extended networks) exhibiting a peculiar bowl-like
topography are evident in atomic force microscopy images. The tall
rims outlining individual domains indicate that squalene accumulates
at the domain periphery in a manner similar to the accumulation of
cholesterol at domain boundaries in their mixtures with phospholipids.
Taken together, the results presented here support the notion that
squalene plays a role in modulating molecular packing and lateral
organization (i.e., domain formation) in the membranes of archaea
analogous to that of cholesterol in eukaryotic membranes