Understanding attraction is difficult enough, but it’s a little less so these days, at least
in amoeba living in the lab of Johns Hopkins scientist Peter Devreotes, Ph.D.

These tiny single-celled organisms have a thing for a certain molecule, called cAMP,
and move toward it with striking efficiency. Similarly, directed movement also guides
human cells in their normal travels and in diseases such as arthritis, asthma,
multiple sclerosis and cancer, says Devreotes. He has been using the amoeba to
examine how this process, known as “chemotaxis,” works.

Now, in the Oct. 26 issue of the journal Science, Japanese scientists and Devreotes
describe success in imaging single molecules of cAMP as they interact with docking
points, or receptors, on the surface of these amoebae. The technique provides
real-time video of how the receptors and cAMP behave. [To view or download video,
see http://www.hopkinsmedicine.org/cellbio/devreotes/Joanna%20Downer.htm].

Copies of the receptor are distributed throughout the amoeba’s outer membrane,
allowing the cell to detect cAMP all around it and even to distinguish which direction
has the highest amounts. Detecting this “gradient” of cAMP, the cell moves constantly
toward higher concentrations of the attractant, says Devreotes, professor and director
of cell biology at the Johns Hopkins School of Medicine’s Institute for Basic
Biomedical Sciences.

“It’s sort of like looking for the ice cream stand at the county fair,” explains Devreotes.
“You may see a few people with ice cream cones, then look around and head off in the
direction more of them seem to be coming from.”

By tagging single molecules of cAMP with a fluorescent dye, the scientists have
obtained images of glowing red spots on living amoebae. Over a period of a few
seconds, the spots, which represent single molecules of cAMP bound to its receptor,
move within the cell membrane before dropping off at random. Among other things,
the images prove that receptors move, or diffuse, within cell membranes, says

“We can see single molecules binding to receptors and actually watch the receptors
move,” says Devreotes. “People know that receptors bind and release molecules, but
until now no one has seen the process one event at a time.”

The images also prove that cAMP binds to receptors throughout the cell membrane. “It
could have been that receptors shifted to the side with the higher concentration of
cAMP,” says Devreotes. “A uniform distribution, however, lets the cell respond faster to
changes in the cAMP gradient.”

Instead of being more numerous, the receptors on the “front” of the cell had faster and
more frequent interactions with cAMP, the images showed. The images also showed
that not all the receptors were constantly or randomly diffusing. The scientists suspect
that apparently stationary receptors or those moving in a straight line might be
interacting with the internal skeleton of the cell.


Authors on the study, in addition to Devreotes, are lead author Masahiro Ueda,
Yasushi Sako, and Toshio Yanagida of physiology and biosignaling at the Graduate
School of Medicine, Osaka University, and Toshiki Tanaka, formerly of the
Biomolecular Engineering Research Institute in Osaka and now in the department of
applied chemistry at Nagoya Institute of Technology.