Introduction
Rhodopsin is the dim-light photoreceptor, and is a prototypical member of the G protein-coupled receptor (GPCR) family (1). Absorption of a photon by the 11-cis-retinal chromophore triggers its isomerization to the all-trans form, converting light energy into atomic motion (2). Rhodopsin then thermally relaxes through a series of distinct photointermediates, each with characteristic UV/Visible absorption maxima (λmax), as shown below (3). An equilibrium is formed between the later photointermediates, metarhodopsin I (meta I, λmax = 480 nm) and metarhodopsin II (meta II, λmax = 380 nm). Meta II corresponds to the fully-activated receptor, which binds to and activates the heterotrimeric G protein transducin (4).
To understand the molecular details of visual transduction, and GPCR activation in general, we need to study the structure of photoactivated states of rhodopsin. We have used UV/Visible and FTIR difference spectroscopy (5, pdf) to characterise the photointermediates formed in p22121 2D crystals. These studies show that rhodopsin forms photointermediates up to meta I within 2D crystals. We have focussed on studying the later photointermediate, meta I, by freeze-trapping and electron cryo-microscopy.