Opsins conservation

A key feature of both vertebrate and invertebrate opsin molecules is their ability to interact with a G protein, typically transducin, to initiate phototransduction (Ebrey Koutalos 2001). The third cytoplasmic loop that connects a-hehces V and VI contributes to the G protein binding and activation and is highly conserved amongst the rod and cone opsins (Fig. 1, Table 2). This conservation extends to the P opsin family. However, the third cytoplasmic loop of the VA... 

Taken together, these observations suggest that whatever the function of melanopsins they do not need to have a highly conserved protein structure, and in this respect differ markedly from the known photosensory opsins. Furthermore, the melanopsins do not share a conserved genomic structure with the photosensory opsins (Fig. 2), suggesting a different evolutionary lineage to the characterised photosensory opsins (Bellingham Foster 2002). 

Genomic structure Conserved (introns 1, 3 and 4) in the vertebrates and chordates No introns conserved with the known opsin families No introns conserved with the known opsin families... 

Foster Absolutely. The point 1 was trying to make is that the known photosensory opsins are highly conserved, whilst the whole opsin family shows considerable divergence. 

The structure of the ligand-free opsin bound to a synthetic peptide derived from the C-terminus of the a-subunit of transducin has recently been obtained [15], This structure has shown that the a5 helix of G t binds to a site in opsin that is opened by the movement of the cytoplasmic end of TM6 away from TM3 and towards TM5 (see above). The C-terminal domain of the G protein interacts with the extended conformation of R3.50, the short loop connecting TM7 and Hx8 and the inner side of the cytoplasmic TMs 5 and 6 (Figure 2.7). Notably, both the G protein family (positions i-2 and i-7 relative to the final amino acid) and TMs 5 (positions 5.61 and 5.65) and 6 (position 6.33) of class A GPCRs contain highly conserved hydrophobic amino adds that form key hydrophobic contacts between the receptor and the G protein. Notably, chemokine receptors also possess hydrophobic amino acids at these 5.61 (I 75% L 10% V 5%), 5.65 (L 90% I 5%) and 6.33 (A 75% L 5%) positions. It, thus, seems reasonable to assume that the mode of recognition of the G protein by the chemokine receptor family resembles this structure found for opsin [15]. 

---