Single molecules of cAMP engage receptors on Dictyostelium.


When cells of the slime mold Dictyostelium sense cAMP, they get the urge to merge, steering toward the source of the chemical and fusing into a slimy blob. By observing labeled cAMP receptors on traveling slime mold cells, Masahiro Ueda (Osaka University, Osaka, Japan) and coworkers were able to document differences in receptor activity between the “front” and “back” ends. The paper marks one of the first uses of a new technique for resolving and monitoring individual molecules.

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 important to determine how cells initially form such a polarity in receptor states and whether chemoattractant gradients can modify it,” Ueda says. ▪


Ueda, M., et al.