Sci. Signal., 30 June 2009
Microbiology Really Random Receptors
L. Bryan Ray
Science, Science Signaling, AAAS, Washington, DC 20005, USA
Bacteria can sense tiny changes in the concentration of food molecules and can adjust their swimming motion to move toward the source of a gradient. They can do this in part because they localize enormous clusters of thousands of transmembrane receptors at specific positions at opposite ends of the cell, the cell poles. Surprisingly, evidence has suggested that formation and maintenance of such clusters may occur through stochastic self-assembly of clusters, rather than by association of receptors with some sort of anchor. But to test this idea rigorously, one needs the ability to detect and count single receptor proteins even if they are densely packed in clusters. Thats a job for PALM (photoactivated localization microscopy), a technique in which fusion proteins containing a photoactivatable moiety are stimulated with a low intensity of UV light that activates a single molecule at a time in a small area monitored in a microscope. This allows optical resolution 10 to 100 times better than the diffraction limit for light microscopy. Greenfield et al. analyzed over 1 million receptor molecules and observed that many were present as single receptors or small clusters, consistent with a stochastic model of cluster assembly. They also extended a mathematical model in which the receptors are randomly inserted in the membrane but can be captured and incorporated into existing clusters and showed that the model could account for the observed distribution of receptors. Because of this capture by existing clusters, the density of new clusters is highest at a position farthest from a large cluster. This, the authors explain, means that through random formation of clusters, a cell with a large cluster at one pole will normally form a new large cluster at the opposite pole. Generation of new membrane occurs in lateral parts of the cell, further favoring clustering of receptors toward the poles. The authors propose that these processes can give rise to distinct large clusters of receptors of appropriate size and stability, all without any specific cellular machinery to position the receptors.
D. Greenfield, A. L. McEvoy, H. Shroff, G. E. Crooks, N. S. Wingreen, E. Betzig, J. Liphardt, Self-organization of the Escherichia coli chemotaxis network imaged with super-resolution light microscopy. PLoS Biol. 7, e1000137 (2009). [PubMed]
Citation: L. B. Ray, Really Random Receptors. Sci. Signal. 2, ec212 (2009).
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