A Hidden Markov Model for Single Particle Tracks Quantifies Dynamic Interactions between LFA-1 and the Actin Cytoskeleton

Raibatak Das,Daniel Coombs,Christopher W Cairo,Rama Ranganathan

doi:10.1371/journal.pcbi.1000556

Abstract

The extraction of hidden information from complex trajectories is a continuing problem in single-particle and single-molecule experiments. Particle trajectories are the result of multiple phenomena, and new methods for revealing changes in molecular processes are needed. We have developed a practical technique that is capable of identifying multiple states of diffusion within experimental trajectories. We model single particle tracks for a membrane-associated protein interacting with a homogeneously distributed binding partner and show that, with certain simplifying assumptions, particle trajectories can be regarded as the outcome of a two-state hidden Markov model. Using simulated trajectories, we demonstrate that this model can be used to identify the key biophysical parameters for such a system, namely the diffusion coefficients of the underlying states, and the rates of transition between them. We use a stochastic optimization scheme to compute maximum likelihood estimates of these parameters. We have applied this analysis to single-particle trajectories of the integrin receptor lymphocyte function-associated antigen-1 (LFA-1) on live T cells. Our analysis reveals that the diffusion of LFA-1 is indeed approximately two-state, and is characterized by large changes in cytoskeletal interactions upon cellular activation.

Highlights

The lateral mobility of cell-surface proteins plays a critical role in mediating the biological functions of membrane proteins [1]
Many important biological processes begin when a target molecule binds to a cell surface receptor protein
Surface receptors are mobile on the cell surface and their mobility is influenced by their interaction with intracellular proteins

Summary

Introduction

The lateral mobility of cell-surface proteins plays a critical role in mediating the biological functions of membrane proteins [1]. A variety of biophysical techniques, fluorescence microscopy experiments, have been extensively utilized to quantify the lateral mobility of membrane proteins. The complementary techniques of single particle tracking FRAP captures the behavior of a population of labeled particles on a spatial scale of a few microns, while SPT records the dynamics of individual molecules or small macromolecular clusters over lengths of tens to hundreds of nanometers. In a typical SPT experiment, a membrane-associated protein is labeled, either fluorescently or with an antibody conjugated bead, and imaged using high speed video microscopy with a temporal resolution of tens of milliseconds or less. The enhanced spatial resolution of SPT, as well as its non-ensemble nature, make the technique attractive for detailed single molecule studies of cell surface receptor dynamics

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Discussion

Conclusion