Measurement of dipolar couplings in a transducin peptide fragment weakly bound to oriented photo-activated rhodopsin

Rhodopsin-containing disks, isolated from rod outer segments of bovine retina, align at high magnetic fields with their membrane normal parallel to the magnetic field. After light-activation of rhodopsin, transient binding of the C-terminal transducin undecapeptide, selectively labeled with N-15 at Leu(5) and Gly(9), results in residual dipolar contributions to the (1)J(NH) splittings for these two residues. Both residues show (1)J(NH) splittings which are smaller than in the dark-adapted or rhodopsin-free sample, and return to their isotropic values at a rate determined by the decay of the meta II state of rhodopsin. The dipolar couplings indicate that in the bound state, N-H vectors of Leu(5) and Gly(9) make angles of 48 +/- 4 degrees and 40 +/- 8 degrees, respectively, with the disk normal. These `transferred' dipolar couplings potentially offer a useful method for studying the conformation and orientation of flexible, low affinity ligands when bound to oriented integral membrane receptors.