Length control of microtubules by depolymerizing motor proteins

In many intracellular processes, the length distribution of microtubules is controlled by depolymerizing motor proteins. Experiments have shown that, following non-specific binding to the surface of a microtubule, depolymerizers are transported to the microtubule tip(s) by diffusion or directed walk and, then, depolymerize the microtubule from the tip(s) after accumulating there. We develop a quantitative model to study the depolymerizing action of such a generic motor protein, and its possible effects on the length distribution of microtubules. We show that, when the motor protein concentration in solution exceeds a critical value, a steady state is reached where the length distribution is, in general, non-monotonic with a single peak. However, for highly processive motors and large motor densities, this distribution effectively becomes an exponential decay. Our findings suggest that such motor proteins may be selectively used by the cell to ensure precise control of MT lengths. The model is also used to analyze experimental observations of motor-induced depolymerization.Comment: Added section with figures and significantly expanded text, current version to appear in Europhys. Let

Comparison of inter-proton distances in DNA models with nuclear overhauser enhancement data

The conformational flexibility inherent in the polynucleotide chain plays an important role in deciding its three-dimensonal structure and enables it to undergo structural transitions in order to fulfil all its functions. Following certain stereochemical guidelines, both right and left handed double-helical models have been built in our laboratory and they are in reasonably good agreement with the fibre patterns for various polymorphous forms of DNA. Recently, nuclear magnetic resonance spectroscopy has become an important technique for studying the solution conformation and polymorphism of nucleic acids. Several workers have used 1H nuclear magnetic resonance nuclear Overhauser enhancement measurements to estimate the interproton distances for the various DNA oligomers and compared them with the-interproton distances for particular models of A and B form DNA. In some cases the solution conformation does not seem to fit either of these models. We have been studying various models for DNA with a view to exploring the full conformational space allowed for nucleic acid polymers. In this paper, the interproton distances calculated for the different stereochemically feasible models of DNA are presented and they are compared and correlated against those obtained from 1H nuclear magnetic resonance nuclear Overhauser enhancement measurements of various nucleic acid oligomers

Gas permeation through a polymer network

We study the diffusion of gas molecules through a two-dimensional network of polymers with the help of Monte Carlo simulations. The polymers are modeled as non-interacting random walks on the bonds of a two-dimensional square lattice, while the gas particles occupy the lattice cells. When a particle attempts to jump to a nearest-neighbor empty cell, it has to overcome an energy barrier which is determined by the number of polymer segments on the bond separating the two cells. We investigate the gas current $J$ as a function of the mean segment density $\rho$, the polymer length $\ell$ and the probability $q^{m}$ for hopping across $m$ segments. Whereas $J$ decreases monotonically with $\rho$ for fixed $\ell$, its behavior for fixed $\rho$ and increasing $\ell$ depends strongly on $q$. For small, non-zero $q$, $J$ appears to increase slowly with $\ell$. In contrast, for $q=0$, it is dominated by the underlying percolation problem and can be non-monotonic. We provide heuristic arguments to put these interesting phenomena into context.Comment: Dedicated to Lothar Schaefer on the occasion of his 60th birthday. 11 pages, 3 figure

Iodine Uptake Restoration in Thyroid Cancer

A method of expressing a tumor specific therapeutic response element in a cancerous cell is disclosed in which the response element was previously blocked from expression. The method comprises the step of administering an unblocking agent to the cancerous cell harboring a gene encoding the response element, thereby resulting in the expression of the response element

Bargaining Theory and Regulatory Reform: The Political Logic of Inefficient Regulation

In this Article David Spence and Lekha Gopalakrishnan pro- pose a new understanding of regulatory bargaining. Economists and others have long argued that the American regulatory system is unnecessarily inefficient. Critics charge that the system is both substantively inefficient, in that it sometimes mandates the use of inefficient means for achieving a regulatory goal, and procedurally inefficient, in its over-reliance on rules. These arguments have led to a wave of regulatory reform experiments in the federal bureaucracy, many of which seek to promote positive-sum changes in regulatory policy through bargaining among private- and public-sector stakeholders. As several commentators have noted, most of these regulatory reforms have not met expectations in that bargaining participants often forgo positive-sum changes in the status quo. Those same commentators have offered a variety of explanations for these failures, most of which are either unpersuasive or incomplete. Spence and Gopalakrishnan propose an another explanation drawn from the standard bargaining literature in economics, one that seems to explain the trajectory of recent regulatory reforms. The authors argue that, in the context of political conflict over policy changes, participants in these bargaining processes view positive-sum policy changes in zero-sum terms. That is, they bargain strategically, using their power to veto these positive-sum changes to extract further policy concessions from other stakeholders. This revelation has important implications for the future of this kind of regulatory reform

Chemisorption of Nitrous Oxide on Transition Metal Surfaces-Bond Energy Bond Order Model Calculation

157-15

Preparation and thermal decomposition of some oxomolybdenum(VI) oxalates

Anionic oxomolybdenum(VI) oxalates having the general formula A2[MoO3(C2O4] · H2O where A = K+ or NH4+ are prepared, characterized by chemical and infrared spectra and their thermal decomposition studied by DTA and TGA. A chain structure containing MoO6 octahedra linked through oxygen is proposed for the complex oxalates on the basis of i.r. absorption spectra. The ammonium compound decomposes endothermally around 270°C to give lower oxides of molybdenum which are finally oxidised to MoO3, while the final decomposition product of the potassium compound is K2MoO4