Structure and function in rhodopsin: mapping light-dependent changes in distance between residue 316 in helix 8 and residues in the sequence 60-75, covering the cytoplasmic end of helices TM1 and TM2 and their connection loop CL1.

Double-spin-labeled mutants of rhodopsin were prepared containing a nitroxide side chain at position 316 in the cytoplasmic surface helix H8, and a second nitroxide in the sequence of residues 60-75, which includes the cytoplasmic loop CL1 and cytoplasmic ends of helices TM1 and TM2. Magnetic dipole-dipole interactions between the spins were analyzed to provide interspin distance distributions in both the dark and photoactivated states of rhodopsin. In the dark state in solutions of dodecyl maltoside, the interspin distances are found to be consistent with structural models of the nitroxide side chain and rhodopsin, both derived from crystallography. Photoactivation of rhodopsin shows a pattern of increases in internitroxide distance between the reference, position 316 in H8, and residues in CL1 and TM2 that suggests an outward displacement of TM2 relative to H8 by approximately 3 A.