To label the individual cAMP receptors, the authors fused cAMP molecules to an orange fluorescent dye called Cy3 and thenexposed the slime mold cells to the compound. Using a total internal reflection fluorescence microscope, they could track and count the receptors on the cell surface that had bound to the glowing cAMP. “This technique can reveal the dynamics of the individual signaling molecules in living cells,” says Ueda.

The observations confirm previous reports that the migrating cells are polarized. Although there were about the same number of receptors at each end, 12% more receptors were bound to cAMP at the anterior end than at the posterior end. The investigators also found that the receptors at the anterior end released their cAMP more quickly. How the cells become polarized remains a mystery, Ueda says. However, polarity apparently develops independent of the cAMP gradient. “It will be...

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