370 research outputs found
Receptor-Mediated Endocytosis of a Cylindrical Nanoparticle in the Presence of Cytoskeleton Substrate
Internalization of particles by cells plays a crucial role for adsorbing
nutrients and fighting infection. Endocytosis is one of the most important
mechanisms of the particles uptake which encompass multiple pathways. Although
endocytosis is a complex mechanism involving biochemical signaling and active
force generation, the energetic cost associated to the large deformations of
the cell membrane wrapping around the foreign particle is an important factor
controlling this process, which can be studied using quantitative physical
models. Of particular interest is the competition between membrane -
cytoskeleton and membrane - target adhesion. Here, we explore the wrapping of a
lipid membrane around a long cylindrical object in the presence of a substrate
mimicking the cytoskeleton. Using discretization of the Helfrich elastic energy
that accounts for the membrane bending rigidity and surface tension, we obtain
a wrapping phase diagram as a function of the membrane-cytoskeleton and the
membrane-target adhesion energy that includes unwrapped, partially wrapped and
fully wrapped states. We provide an analytical expression for the boundary
between the different regimes. While the transition to partial wrapping is
independent of membrane tension, the transition to full wrapping is very much
influenced by membrane tension. We also show that target wrapping may proceed
in an asymmetric fashion in the full wrapping regime
Theory of Nucleosome Corkscrew Sliding in the Presence of Synthetic DNA Ligands
Histone octamers show a heat-induced mobility along DNA. Recent theoretical
studies have established two mechanisms that are qualitatively and
quantitatively compatible with in vitro experiments on nucleosome sliding:
Octamer repositiong through one-basepair twist defects and through ten-basepair
bulge defects. A recent experiment demonstrated that the repositioning is
strongly suppressed in the presence of minor-groove binding DNA ligands. In the
present study we give a quantitative theory for nucleosome repositioning in the
presence of such ligands. We show that the experimentally observed octamer
mobilities are consistent with the picture of bound ligands blocking the
passage of twist defects through the nucleosome. This strongly supports the
model of twist defects inducing a corkscrew motion of the nucleosome as the
underlying mechanism of nucleosome sliding. We provide a theoretical estimate
of the nucleosomal mobility without adjustable parameters, as a function of
ligand concentration, binding affinity, binding site orientiation, temperature
and DNA anisotropy. Having this mobility at hand we speculate about the
interaction between a nucleosome and a transcribing RNA polymerase and suggest
a novel mechanism that might account for polymerase induced nucleosome
repositioning.Comment: 23 pages, 4 figures, submitted to J. Mol. Bio
- …