SignificanceThe multifaceted involvement of GM1 as a ligand in many cellular functions has been well recognized. We find that GM1 readily desorbs from the membrane of cell-sized model biomimetic systems (giant unilamellar vesicles). The desorption is unbalanced, creating an asymmetry between the bilayer leaflets. This results in reshaping weakly curved membranes into nanotubular invaginations stabilized by the membrane spontaneous curvature, which we quantify experimentally. Computer simulations confirm the experimental results. Uncovering the role of GM1 as a fine regulator of membrane curvature broadens our perspective on its important function in reshaping neuronal membranes and emphasizes that GM1 desorption can strongly affect the cell membrane morphology.
Biological
membranes possess intrinsic asymmetry. This asymmetry
is associated not only with leaflet composition in terms of membrane
species but also with differences in the cytosolic and periplasmic
solutions containing macromolecules and ions. There has been a long
quest for understanding the effect of ions on the physical and morphological
properties of membranes. Here, we elucidate the changes in the mechanical properties of membranes exposed
to asymmetric buffer conditions and the associated curvature generation.
As a model system, we used giant unilamellar vesicles (GUVs) with
asymmetric salt and sugar solutions on the two sides of the membrane.
We aspirated the GUVs into micropipettes and attached small beads
to their membranes. An optical tweezer was used to exert a local force
on a bead, thereby pulling out a membrane tube from the vesicle. The
assay allowed us to measure the spontaneous curvature and the bending
rigidity of the bilayer in the presence of different ions and sugar.
At low sugar/salt (inside/out) concentrations, the membrane spontaneous
curvature generated by NaCl and KCl is close to zero, but negative
in the presence of LiCl. In the latter case, the membrane bulges away
from the salt solution. At high sugar/salt conditions, the membranes
were observed to become more flexible and the spontaneous curvature
was enhanced to even more negative values, comparable to those generated
by some proteins. Our findings reveal the reshaping role of alkali
chlorides on biomembranes.
Near-infrared laser (785-nm)-excited Raman spectra from a red blood cell, optically trapped using the same laser beam, show significant changes as a function of trapping duration even at trapping power level of a few milliwatts. These changes in the Raman spectra and the bright-field images of the trapped cell, which show a gradual accumulation of the cell mass at the trap focus, suggest photoinduced aggregation of intracellular heme. The possible role of photoinduced protein denaturation and hemichrome formation in the observed aggregation of heme is discussed.
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