Fig. 3.

Illustration of two molecules of Cy3B-labeled M₁ receptors reversibly forming a dimer. One hundred sequential frames (30-ms exposure time/frame) of two molecules of Cy3B-labeled M₁ receptors diffusing on the surface of a CHO cell. These molecules are initially monomers (about the first 21 frames), form a dimer (single red spot) for ∼40 frames (∼1 s), and then dissociate into monomers, possibly for the rest of the recording. There may be a transient reformation of short lifetime dimers for about three frames in the latter part of the recording, but it is not possible to exclude this being a chance overlap of the images of two molecules that are not interacting but diffusing independently close to each other. (Lower) A color look-up table. Two-color imaging (see text and Fig. 4) was used to study the details of M₁ receptor dimerization.