Combined biophysical and biochemical information confirms arrangement of transmembrane helices visible from the three-dimensional map of frog rhodopsin

The electron density projection map of frog rhodopsin at 6 A resolution had been until recently the most direct evidence for the three-dimensional structure of a transmembrane domain of any G-protein-coupled receptor. Only three out of seven transmembrane helices are clearly defined, whilst the other four are hidden in a patch of unresolved electron density. A model of the seven-helix bundle has been created by generating positions and orientations for the four unresolved helices through performing a conformational search directed by structural restraints derived from other experimental data. These four helices are significantly tilted with respect to the membrane normal, and the cytosolic end of helix C is inserted between helices D and E. These calculations produce positions and orientations for these additional helices that are consistent with the recently published low-resolution three-dimensional map, and provide a template for more detailed modelling of rhodopsin structure and function.