239 research outputs found
Transition States in Protein Folding Kinetics: The Structural Interpretation of Phi-values
Phi-values are experimental measures of the effects of mutations on the
folding kinetics of a protein. A central question is which structural
information Phi-values contain about the transition state of folding.
Traditionally, a Phi-value is interpreted as the 'nativeness' of a mutated
residue in the transition state. However, this interpretation is often
problematic because it assumes a linear relation between the nativeness of the
residue and its free-energy contribution. We present here a better structural
interpretation of Phi-values for mutations within a given helix. Our
interpretation is based on a simple physical model that distinguishes between
secondary and tertiary free-energy contributions of helical residues. From a
linear fit of our model to the experimental data, we obtain two structural
parameters: the extent of helix formation in the transition state, and the
nativeness of tertiary interactions in the transition state. We apply our model
to all proteins with well-characterized helices for which more than 10
Phi-values are available: protein A, CI2, and protein L. The model captures
nonclassical Phi-values 1 in these helices, and explains how different
mutations at a given site can lead to different Phi-values.Comment: 26 pages, 7 figures, 5 table
Cooperativity in two-state protein folding kinetics
We present a solvable model that predicts the folding kinetics of two-state
proteins from their native structures. The model is based on conditional chain
entropies. It assumes that folding processes are dominated by small-loop
closure events that can be inferred from native structures. For CI2, the src
SH3 domain, TNfn3, and protein L, the model reproduces two-state kinetics, and
it predicts well the average Phi-values for secondary structures. The barrier
to folding is the formation of predominantly local structures such as helices
and hairpins, which are needed to bring nonlocal pairs of amino acids into
contact.Comment: 9 pages, 6 figures, 1 tabl
Adhesion-induced phase separation of multiple species of membrane junctions
A theory is presented for the membrane junction separation induced by the
adhesion between two biomimetic membranes that contain two different types of
anchored junctions (receptor/ligand complexes). The analysis shows that several
mechanisms contribute to the membrane junction separation. These mechanisms
include (i) the height difference between type-1 and type-2 junctions is the
main factor which drives the junction separation, (ii) when type-1 and type-2
junctions have different rigidities against stretch and compression, the
``softer'' junctions are the ``favored'' species, and the aggregation of the
softer junction can occur, (iii) the elasticity of the membranes mediates a
non-local interaction between the junctions, (iv) the thermally activated shape
fluctuations of the membranes also contribute to the junction separation by
inducing another non-local interaction between the junctions and renormalizing
the binding energy of the junctions. The combined effect of these mechanisms is
that when junction separation occurs, the system separates into two domains
with different relative and total junction densities.Comment: 23 pages, 6 figure
Segregation of receptor-ligand complexes in cell adhesion zones: Phase diagrams and role of thermal membrane roughness
The adhesion zone of immune cells, the 'immunological synapse', exhibits
characteristic domains of receptor-ligand complexes. The domain formation is
likely caused by a length difference of the receptor-ligand complexes, and has
been investigated in experiments in which T cells adhere to supported membranes
with anchored ligands. For supported membranes with two types of anchored
ligands, MHCp and ICAM1, that bind to the receptors TCR and LFA1 in the cell
membrane, the coexistence of domains of TCR-MHCp and LFA1-ICAM1 complexes in
the cell adhesion zone has been observed for a wide range of ligand
concentrations and affinities. For supported membranes with long and short
ligands that bind to the same cell receptor CD2, in contrast, domain
coexistence has been observed for a rather narrow ratio of ligand
concentrations. In this article, we determine detailed phase diagrams for cells
adhering to supported membranes with a statistical-physical model of cell
adhesion. We find a characteristic difference between the adhesion scenarios in
which two types of ligands in a supported membrane bind (i) to the same cell
receptor or (ii) to two different cell receptors, which helps to explain the
experimental observations. Our phase diagrams fully include thermal shape
fluctuations of the cell membranes on nanometer scales, which lead to a
critical point for the domain formation and to a cooperative binding of the
receptors and ligands.Comment: 23 pages, 6 figure
The linear tearing instability in three dimensional, toroidal gyrokinetic simulations
Linear gyro-kinetic simulations of the classical tearing mode in
three-dimensional toroidal geometry were performed using the global gyro
kinetic turbulence code, GKW . The results were benchmarked against a
cylindrical ideal MHD and analytical theory calculations. The stability, growth
rate and frequency of the mode were investigated by varying the current
profile, collisionality and the pressure gradients. Both collision-less and
semi-collisional tearing modes were found with a smooth transition between the
two. A residual, finite, rotation frequency of the mode even in the absense of
a pressure gradient is observed which is attributed to toroidal finite
Larmor-radius effects. When a pressure gradient is present at low
collisionality, the mode rotates at the expected electron diamagnetic
frequency. However the island rotation reverses direction at high
collisionality. The growth rate is found to follow a scaling with
collisional resistivity in the semi-collisional regime, closely following the
semi-collisional scaling found by Fitzpatrick. The stability of the mode
closely follows the stability using resistive MHD theory, however a
modification due to toroidal coupling and pressure effects is seen
Phi-values in protein folding kinetics have energetic and structural components
Phi-values are experimental measures of how the kinetics of protein folding
is changed by single-site mutations. Phi-values measure energetic quantities,
but are often interpreted in terms of the structures of the transition state
ensemble. Here we describe a simple analytical model of the folding kinetics in
terms of the formation of protein substructures. The model shows that
Phi-values have both structural and energetic components. In addition, it
provides a natural and general interpretation of "nonclassical" Phi-values
(i.e., less than zero, or greater than one). The model reproduces the
Phi-values for 20 single-residue mutations in the alpha-helix of the protein
CI2, including several nonclassical Phi-values, in good agreement with
experiments.Comment: 15 pages, 3 figures, 1 tabl
Fluctuation induced interactions between domains in membranes
We study a model lipid bilayer composed of a mixture of two incompatible
lipid types which have a natural tendency to segregate in the absence of
membrane fluctuations. The membrane is mechanically characterized by a local
bending rigidity which varies with the average local lipid
composition . We show, in the case where varies weakly with
, that the effective interaction between lipids of the same type can
either be everywhere attractive or can have a repulsive component at
intermediate distances greater than the typical lipid size. When this
interaction has a repulsive component, it can prevent macro-phase separation
and lead to separation in mesophases with a finite domain size. This effect
could be relevant to certain experimental and numerical observations of
mesoscopic domains in such systems.Comment: 9 pages RevTex, 1 eps figur
Dynamic phase separation of fluid membranes with rigid inclusions
Membrane shape fluctuations induce attractive interactions between rigid
inclusions. Previous analytical studies showed that the fluctuation-induced
pair interactions are rather small compared to thermal energies, but also that
multi-body interactions cannot be neglected. In this article, it is shown
numerically that shape fluctuations indeed lead to the dynamic separation of
the membrane into phases with different inclusion concentrations. The tendency
of lateral phase separation strongly increases with the inclusion size. Large
inclusions aggregate at very small inclusion concentrations and for relatively
small values of the inclusions' elastic modulus.Comment: 6 pages, 6 figure
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