posted on 2013-01-30, 00:00authored byChristine
S. Scheve, Paul A. Gonzales, Noor Momin, Jeanne C. Stachowiak
Cellular membranes are densely crowded with a diverse
population
of integral and membrane-associated proteins. In this complex environment,
lipid rafts, which are phase-separated membrane domains enriched in
cholesterol and saturated lipids, are thought to organize the membrane
surface. Specifically, rafts may help to concentrate proteins and
lipids locally, enabling cellular processes such as assembly of caveolae,
budding of enveloped viruses, and sorting of lipids and proteins in
the Golgi. However, the ability of rafts to concentrate protein species
has not been quantified experimentally. Here we show that when membrane-bound
proteins become densely crowded within liquid-ordered membrane regions,
steric pressure arising from collisions between proteins can destabilize
lipid phase separations, resulting in a homogeneous distribution of
proteins and lipids over the membrane surface. Using a reconstituted
system of lipid vesicles and recombinant proteins, we demonstrate
that protein–protein steric pressure creates an energetic barrier
to the stability of phase-separated membrane domains that increases
in significance as the molecular weight of the proteins increases.
Comparison with a simple analytical model reveals that domains are
destabilized when the steric pressure exceeds the approximate enthalpy
of membrane mixing. These results suggest that a subtle balance of
free energies governs the stability of phase-separated cellular membranes,
providing a new perspective on the role of lipid rafts as concentrators
of membrane proteins.