Effects of receptor clustering on ligand dissociation Theory

This un-edited manuscript has been accepted for publication in Biophysical

Journal and is freely available on BioFast at 82006b4f2e3f5727a5e9623c. The final

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Effects of receptor clustering on ligand dissociation kinetics: Theory and simulations

Manoj Gopalakrishnan()2,1, Kimberly Forsten-Williams4()3, Matthew A. Nugent()4and Uwe C. T?uber()2

1 Department of Biological Physics5,

Max-Planck-Institut für Physik komplexer Systeme,

N?thnitzer Stra e 38,

01187 Dresden, Germany.

2Department of Physics and Center for Stochastic Processes in Science and Engineering,

Virginia Polytechnic Institute and State University,

Blacksburg, VA 24061, USA.

3 Department of Chemical Engineering

and Virginia Tech - Wake Forest University School of Biomedical Engineering and Sciences,

Virginia Polytechnic Institute and State University,

Blacksburg, VA 24061, USA.

4 Department of Biochemistry,

Boston University School of Medicine,

Boston, MA 02118, USA.

Key words: Stochastic theory, Rebinding, Receptors,

fibroblast growth factor-2 (FGF-2, bFGF), Monte Carlo simulations. Running Title: Clustering Effect on Ligand Dissociation

4 Corresponding author: e-mail: kfw@82006b4f2e3f5727a5e9623c

5 Present address

1

Abstract

Receptor-ligand binding is a critical first step in signal transduction and the duration of the interaction can impact signal generation. In mammalian cells, clustering of receptors may be facilitated by heterogeneous zones of lipids, known as lipid rafts. In vitro experiments show that disruption of rafts significantly alters the dissociation of fibroblast growth factor-2 (FGF-2) from heparan sulfate proteoglycans (HSPG), co-receptors for FGF-2. In this paper, we develop a continuum stochastic formalism in order to address how receptor clustering might influence ligand rebinding. We find that clusters reduce the effective dissociation rate dramatically when the clusters are dense and the overall surface density of receptors is low. The effect is much less pronounced in the case of high receptor density and shows non-monotonic behavior with time. These predictions are verified via lattice Monte Carlo simulations. Comparison with FGF-2-HSPG experimental results is made and suggests that the theory could be used to analyze similar biological systems. We further present an analysis of an additional co-operative “internal diffusion” model that might be used by other systems to increase ligand retention when simple rebinding is insufficient.

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(I) INTRODUCTION

The cell membrane is composed of many different types of lipid species. This heterogeneity leads to the possibility of organization of different species into distinct domains (1). Such domains are especially suited and designed for specialized functions such as signal transduction, nutrient adsorption, and endocytosis. They can link specific cellular machinery and physical features and are equipped with mechanisms for maintenance (addition and removal of specific molecules) for a certain period of time, during which the domains may diffuse as single entities (2). Lipid rafts, which are micro-domains rich in sphingolipids and cholesterol, represent one of the most interesting but insufficiently understood lipid domains (3). Various estimates are available for raft sizes, and diameters in the range 25-200 nm have been reported using various methods (4). A limitation in this area remains that the definition of lipid rafts is rather broad and currently includes a wide range of what will likely prove to be distinct domains that may be distinguished by the particular protein and lipid compositions (2,4,5). Operational definitions of rafts based on resistance to detergent solubilization and sensitivity to cholesterol removal are limited by artifacts of the various procedures used to define rafts and on difficulties in relating model membranes to cell membranes. Nonetheless, it is clear that cell membranes are not homogeneous and that protein-protein, protein-lipid and lipid-lipid interactions all participate in regulating raft size, dynamics and function. Consequently, a myriad of functions have been prescribed to lipid rafts; one possibility being that lipid rafts may serve as mediators of signal transduction for several growth factors, including fibroblast growth factor-2 (FGF-2) (6-8).

Growth factors act as triggers for many cellular processes and their actions are typically mediated by binding of ligand to the extracellular domain of transmembrane receptor proteins. For many receptors, signal transduction requires dimerization or clustering whereby two or more receptors, following ligand binding, interact directly to facilitate signal transduction. While ligand binding is generally specific to members of a family of transmembrane receptor proteins, heparin-binding growth factors such as FGF-2 interact with both specific members of the FGF receptor family and heparan sulfate glycosaminoglycan chains of cell surface proteoglycans (HSPG). HSPG represent a varied class of molecules, including the transmembrane syndecans, the glycosyl-phosphotidylinositol anchored glypicans, and extracellular proteoglycans such as perlecan (reviewed in 9,10). The interaction of FGF-2 with HSPG is of a lower affinity than to the cell surface signaling receptor (CSR) but has been shown to stabilize FGF-2-CSR binding and activation of CSR (11,12). Moreover, HSPG have recently been demonstrated to function directly as signaling receptors in response to FGF-2 binding, leading to the activation of protein kinase C alpha (12) and Erk1/2 (6).

There is evidence that cell surface HSPG are not distributed uniformly, but are instead localized in lipid rafts (6,14-16), and this association may be facilit …… 此处隐藏：52739字，全部文档内容请下载后查看。喜欢就下载吧 ……