Change of Line Tension
in Phase-Separated Vesicles
upon Protein Binding
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Abstract
We measured the effect of a model membrane-binding protein
on line
tension and morphology of phase-separated lipid-bilayer vesicles.
We studied giant unilamellar vesicles composed of a cholesterol/dioleoylphosphatidylcholine/palmitoylsphingomyelin
mixture and a controlled mole fraction of a Ni-chelating lipid. These
vesicles exhibited two coexisting fluid-phase domains at room temperature.
Owing to the line tension, σ, between the two phases, the boundary
between them was pulled like a purse string so that the smaller domain
formed a bud. While observing the vesicles in a microscope, histidine-tagged
green fluorescent protein was added, which bound to the Ni-chelating
lipid. As protein bound, the vesicle shape changed and the length
of the phase boundary increased. The change in morphology was attributed
to a reduction of σ between the two phases because of preferential
accumulation of histidine-tagged green fluorescent protein–Ni-chelating
lipid clusters at the domain boundary. Greater reductions of σ
were found in samples with higher concentrations of Ni-chelating lipid;
this trend provided an estimate of the binding energy at the boundary,
approximately <i>k</i><sub>B</sub><i>T</i>. The
results show how domain boundaries can lead to an accumulation of
membrane-binding proteins at their boundaries and, in turn, how proteins
can alter line tension and vesicle morphology