Figure 1.

Proposed model for first complex formed in photoactivation. Following absorption of light and photoisomerization by one subunit of the rhodopsin dimer, a heterodimer of rhodopsin and Metarhodopsin II (R*) is formed, which quickly complexes with the G protein, transducin, in its heterotrimeric form. A conformational change within G_αt allows release of bound GDP, which then allows binding of GTP and dissociation of activated G_αt–GTP. The representation of the complex is purely schematic, as only indirect low resolution information is available on the structures of the actual complexes. The major contacts with R* are provided by G_αt, with both its carboxyl and fatty acylated amino termini known to be involved. G_βγ are also important for R* binding, with potential interactions with the membrane hydrocarbon phase provided by the farnesyl group on G_γ. For making the structure images shown, PDB files 1U19.pdb(Okada, et al., 2004) and 1GOT.pdb(Lambright, et al., 1996) were used with UCSF Chimera (Meng, et al., 2006, Pettersen, et al., 2004).