Conformational changes are thought to underlie the activation of heter
otrimeric GTP-binding protein (G protein)-coupled receptors. Such chan
ges in rhodopsin were explored by construction of double cysteine muta
nts, each containing one cysteine at the cytoplasmic end of helix C an
d one cysteine at various positions in the cytoplasmic end of helix F.
Magnetic dipolar interactions between spin labels attached to these r
esidues revealed their proximity, and changes in their interaction upo
n rhodopsin light activation suggested a rigid body movement of helice
s relative to one another. Disulfide crosslinking of the helices preve
nted activation of transducin, which suggests the importance of this m
ovement for activation of rhodopsin.