432 research outputs found
Desmoplakin: an unexpected regulator of microtubule organization in the epidermis
Despite their importance in cell shape and polarity generation, the organization of microtubules in differentiated cells and tissues remains relatively unexplored in mammals. We generated transgenic mice in which the epidermis expresses a fluorescently labeled microtubule-binding protein and show that in epidermis and in cultured keratinocytes, microtubules stereotypically reorganize as they differentiate. In basal cells, microtubules form a cytoplasmic network emanating from an apical centrosome. In suprabasal cells, microtubules concentrate at cell–cell junctions. The centrosome retains its ability to nucleate microtubules in differentiated cells, but no longer anchors them. During epidermal differentiation, ninein, which is a centrosomal protein required for microtubule anchoring (Dammermann, A., and A. Merdes. 2002. J. Cell Biol. 159:255–266; Delgehyr, N., J. Sillibourne, and M. Bornens. 2005. J. Cell Sci. 118:1565–1575; Mogensen, M.M., A. Malik, M. Piel, V. Bouckson-Castaing, and M. Bornens. 2000. J. Cell Sci. 113:3013–3023), is lost from the centrosome and is recruited to desmosomes by desmoplakin (DP). Loss of DP prevents accumulation of cortical microtubules in vivo and in vitro. Our work uncovers a differentiation-specific rearrangement of the microtubule cytoskeleton in epidermis, and defines an essential role for DP in the process
Boundary states for WZW models
The boundary states for a certain class of WZW models are determined. The
models include all modular invariants that are associated to a symmetry of the
unextended Dynkin diagram. Explicit formulae for the boundary state
coefficients are given in each case, and a number of properties of the
corresponding NIM-reps are derived.Comment: 34 pages, harvmac (b), 4 eps-figures. One reference added; some minor
typos, as well as the embedding into , are correcte
Coordinating cytoskeletal tracks to polarize cellular movements
For many years after the discovery of actin filaments and microtubules, it was widely assumed that their polymerization, organization, and functions were largely distinct. However, in recent years it has become increasingly apparent that coordinated interactions between microtubules and filamentous actin are involved in many polarized processes, including cell shape, mitotic spindle orientation, motility, growth cone guidance, and wound healing. In the past few years, significant strides have been made in unraveling the intricacies that govern these intertwined cytoskeletal rearrangements
Comments on nonunitary conformal field theories
As is well-known, nonunitary RCFTs are distinguished from unitary ones in a
number of ways, two of which are that the vacuum 0 doesn't have minimal
conformal weight, and that the vacuum column of the modular S matrix isn't
positive. However there is another primary field, call it o, which has minimal
weight and has positive S column. We find that often there is a precise and
useful relationship, which we call the Galois shuffle, between primary o and
the vacuum; among other things this can explain why (like the vacuum) its
multiplicity in the full RCFT should be 1. As examples we consider the minimal
WSU(N) models. We conclude with some comments on fractional level admissible
representations of affine algebras. As an immediate consequence of our
analysis, we get the classification of an infinite family of nonunitary WSU(3)
minimal models in the bulk.Comment: 24 page
The charges of a twisted brane
The charges of the twisted D-branes of certain WZW models are determined. The
twisted D-branes are labelled by twisted representations of the affine algebra,
and their charge is simply the ground state multiplicity of the twisted
representation. It is shown that the resulting charge group is isomorphic to
the charge group of the untwisted branes, as had been anticipated from a
K-theory calculation. Our arguments rely on a number of non-trivial Lie
theoretic identities.Comment: 27 pages, 1 figure, harvmac (b
Charges of Exceptionally Twisted Branes
The charges of the exceptionally twisted (D4 with triality and E6 with charge
conjugation) D-branes of WZW models are determined from the microscopic/CFT
point of view. The branes are labeled by twisted representations of the affine
algebra, and their charge is determined to be the ground state multiplicity of
the twisted representation. It is explicitly shown using Lie theory that the
charge groups of these twisted branes are the same as those of the untwisted
ones, confirming the macroscopic K-theoretic calculation. A key ingredient in
our proof is that, surprisingly, the G2 and F4 Weyl dimensions see the simple
currents of A2 and D4, respectively.Comment: 19 pages, 2 figures, LaTex2e, complete proofs of all statements,
updated bibliograph
Finite Group Modular Data
In a remarkable variety of contexts appears the modular data associated to
finite groups. And yet, compared to the well-understood affine algebra modular
data, the general properties of this finite group modular data has been poorly
explored. In this paper we undergo such a study. We identify some senses in
which the finite group data is similar to, and different from, the affine data.
We also consider the data arising from a cohomological twist, and write down,
explicitly in terms of quantities associated directly with the finite group,
the modular S and T matrices for a general twist, for what appears to be the
first time in print.Comment: 38 pp, latex; 5 references added, "questions" section touched-u
The Rank four Heterotic Modular Invariant Partition Functions
In this paper, we develop several general techniques to investigate modular
invariants of conformal field theories whose algebras of the holomorphic and
anti-holomorphic sectors are different. As an application, we find all such
``heterotic'' WZNW physical invariants of (horizontal) rank four: there are
exactly seven of these, two of which seem to be new. Previously, only those of
rank have been completely classified. We also find all physical modular
invariants for , for , and ,
, completing the classification of ref.{} \SUSU.Comment: 25 pp., plain te
Degradation of a quantum directional reference frame as a random walk
We investigate if the degradation of a quantum directional reference frame
through repeated use can be modeled as a classical direction undergoing a
random walk on a sphere. We demonstrate that the behaviour of the fidelity for
a degrading quantum directional reference frame, defined as the average
probability of correctly determining the orientation of a test system, can be
fit precisely using such a model. Physically, the mechanism for the random walk
is the uncontrollable back-action on the reference frame due to its use in a
measurement of the direction of another system. However, we find that the
magnitude of the step size of this random walk is not given by our classical
model and must be determined from the full quantum description.Comment: 5 pages, no figures. Comments are welcome. v2: several changes to
clarify the key results. v3: journal reference added, acknowledgements and
references update
D-brane charges on non-simply connected groups
The maximally symmetric D-branes of string theory on the non-simply connected
Lie group SU(n)/Z_d are analysed using conformal field theory methods, and
their charges are determined. Unlike the well understood case for simply
connected groups, the charge equations do not determine the charges uniquely,
and the charge group associated to these D-branes is therefore in general not
cyclic. The precise structure of the charge group depends on some number
theoretic properties of n, d, and the level of the underlying affine algebra k.
The examples of SO(3)=SU(2)/Z_2 and SU(3)/Z_3 are worked out in detail, and the
charge groups for SU(n)/Z_d at most levels k are determined explicitly.Comment: 31 pages, 1 figure. 2 refs added. Added the observation: the charge
group for each su(2) theory equals the centre of corresponding A-D-E grou
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