Introduction

The visual pigment rhodopsin is found in disc membranes of the outer segment of rod cells. The rod cells, in conjunction with neurons, are responsible for the visualisation in dim light. Light induces a conformational change of rhodopsin’s chromophore 11-cis-retinal which isomerizes to the all-trans conformation. This triggers a signal transduction cascade including the activation of the G-protein transducin and subsequently the activation of a phosphodiesterase leading to the closure of cGMP-dependent channels in the plasma membrane of the rod outer segment (1). The resulting hyperpolarization of the membrane lowers the rate of transmitter release in the synaptic part of the photoreceptor cell, triggering downstream responses in bipolar cells of the retina.

Structural investigations on two-dimensional crystals of rhodopsin showed that three of the seven transmembrane helices are oriented nearly perpendicular to the membrane plane. The remaining four helices appear to be tilted within the membrane, but so far their exact positions throughout the membrane could not be resolved (2-6). Amino-acid sequence analysis of several hundred G-protein coupled receptors revealed that the orientation and position of the seven transmembrane helices of rhodopsin, and hence the structure of G-protein coupled receptors in general, should be the same (7-9).

Here we present calculations carried out on data collected from recently obtained new two-dimensional crystals of bovine rhodopsin showing p22121 symmetry. For the first time electron diffraction patterns could be taken. With the method of electron cryo-microscopy, images were obtained and an improved projection density map was calculated (20).