REQUIREMENT OF RIGID-BODY MOTION OF TRANSMEMBRANE HELICES FOR LIGHT ACTIVATION OF RHODOPSIN

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.