Divergent mechanisms for the tuning of shortwave sensitive visual pigments in vertebrates David M. Hunt,* Jill A. Cowing, Susan E. Wilkie, Juliet W. L. Parry, Subathra Poopalasundaram and James K. Bowmaker Institute of Ophthalmology, University College London, Bath Street, London, UK EC1V 9EL. E-mail: d.hunt@ucl.ac.uk; Fax: (0)20 7608 6863; Tel: (0)20 7608 6820 Received 19th November 2003, Accepted 2nd March 2004 First published as an Advance Article on the web 22nd March 2004 Of the four classes of vertebrate cone visual pigments, the shortwave-sensitive SWS1 class shows the shortest  max values with peaks in different species in either the violet (390–435 nm) or ultraviolet (around 365 nm) regions of the spectrum. Phylogenetic evidence indicates that the ances- tral pigment was probably UV-sensitive (UVS) and that the shifts between violet and UV have occurred many times during evolution. This is supported by the different mech- anisms for these shifts in different species. All visual pigments possess a chromophore linked via a Schiff base to a Lys residue in opsin protein. In violet-sensitive (VS) pigments, the Schiff base is protonated whereas in UVS pigments, it is almost certainly unprotonated. The gener- ation of VS from ancestral UVS pigments most likely involved amino acid substitutions in the opsin protein that serve to stabilise protonation. The key residues in the opsin protein for this are at sites 86 and 90 that are adjacent to the Schiff base and the counterion at Glu113. In this review, the different molecular mechanisms for the UV or violet shifts are presented and discussed in the context of the structural model of bovine rhodopsin. Introduction Visual pigments are members of the large super-family of G protein coupled receptors which function through the activ- ation of a guanine nucleotide binding protein, the G protein. They are based on a common basic structure of an opsin protein covalently attached to a chromophore via a Schiff base linkage and a conserved lysine residue. Each pigment shows a characteristic peak of maximal absorbance (λ max ), the precise location of this peak depending on inter- actions between the chromophore and the opsin protein. In vertebrates, the chromophore is either 11-cis-retinal or 11-cis-3,4-dehydroretinal, the derivatives of vitamins A1 and A2 respectively, to give either rhodopsin or porphyropsin pigments. The opsin protein in vertebrates consists of a single polypeptide chain of 340–370 amino acids that forms seven α-helical transmembrane (TM) regions connected by cytoplasmic and luminal loops. 1,2 In the tertiary structure, the seven TM regions form a bundle within the membrane creating a cavity towards the luminal side for the chromo- phore. 2 David Hunt graduated with a BSc in Zoology in 1964 followed by a PhD in Genetics in 1967, both from the University of Sheffield. After several moves, he took a lectureship in 1972 at Queen Mary College, University of London, but did not begin work in vision until 1988. He moved to the Institute of Ophthalmology in 1992 where he became Professor of Molecular Genetics in 1996. His research interests range from functional studies of inherited retinal diseases to the molecular evolution of visual pigments. Jill Cowing obtained a BTEC/HNC in Applied Biology in 1987. She joined the Institute of Ophthalmology in 1992 working with David Hunt and has provided technical support in molecular biology to many of the projects undertaken within the research group. She gained a PhD in 2001 on aspects of spectral tuning and evolution of visual pigments in fish. Jill splits her time between research and departmental administration and now holds a Senior Administration post at the Institute. Susan Wilkie graduated in Natural Sciences (Chemistry) from Oxford University in 1972. After several years away from science she returned to take an MSc in Biotechnology at the University of Hertfordshire in 1988, followed by a PhD on DNA fingerprinting of Allium species in 1992. She joined the Institute of Ophthalmology in 1994 where she is now a Senior Research Fellow working on spectral tuning of visual pigments and on functional analysis of proteins linked to retinal eye disease. David Hunt Jill Cowing Susan Wilkie DOI: 10.1039/ b314693f This journal is © The Royal Society of Chemistry and Owner Societies 2004 Photochem. Photobiol. Sci. , 2004, 3, 713–720 713