Rhodopsin, photoreceptor of the rod cell. An emerging pattern for structure and function

Constitutive activation of opsin: influence of charge at position 134 and size at position 296

In previous studies, mutation of Lys296 or Glu113 in opsin has been shown to result in constitutive activation of the protein--that is, these mutants can activate the G protein transducin in the absence of chromophore and in the absence of light. These and other data have led to the suggestion that a salt bridge between Lys296 and Glu113 helps to constrain opsin to an inactive conformation. It is shown here that of 12 different amino acids substituted at position 296, all, except Arg and the wild-type Lys, are constitutively active at neutral pH, lending further support to this suggestion. However, activation of opsin appears also to be influenced significantly by the size of amino acid side chain at position 296. Thus, there are multiple effects of the mutations. Wild-type opsin is also shown to be weakly active at pH 6.1. Five other charged amino acids in the membrane-embedded region of the protein (Asp83, Glu122, Glu134, Arg135, and Glu201) were mutated to see if they affect constitutive activity. Of these amino acids, only mutation of Glu134 results in an increase in the activity of opsin. Changing Glu134 to Gln increases the activity of opsin, while changing Glu134 to Asp inhibits activity. These results suggest that a negative charge on Glu134 is important in stabilizing the inactive state of opsin. Glu134 is highly conserved in all visual pigments and most of the other G protein-linked receptors.

Humanized antibodies

Hybridoma technology enabled rodent monoclonal antibodies to be created against human pathogens and cells, but these had limited clinical utility. Protein engineering is now generating antibodies for treatment of infectious disease, autoimmune disease and cancer by 'humanizing' rodent antibodies. Humanized antibodies have improved pharmacokinetics, reduced immunogenicity and have been used to clinical advantage.

Humanized antibodies

Hybridoma technology enabled rodent monoclonal antibodies to be created against human pathogens and cells, but these had limited clinical utility. Protein engineering, reviewed her by Greg Winter and William Harris, is now generating antibodies for treatment of infectious disease, autoimmune disease and cancer by 'humanizing' rodent antibodies. Humanized antibodies have improved pharmacokinetics, reduced immunogenicity and have been used to clinical advantage.

Projection structure of rhodopsin

Light absorption by the visual pigment rhodopsin triggers, through G-protein coupling, a cascade of events in the outer segment of the rod cell of the vertebrate retina that results in membrane hyperpolarization and nerve excitation. Rhodopsin, which contains 348 amino acids, has seven helices that cross the disk membrane and its amino terminus is extracellular. A wealth of biochemical data is available for rhodopsin: 11-cis retinal is bound to lysine 296 in helix VII; glutamic acid 113 on helix III is the counterion to the protonated Schiff's base; a disulphide bridge, cystine 110-187, connects helix III to the second extracellular loop e2 (refs 13, 14); the carboxy terminus has two palmitoylated cysteines forming a cytoplasmic loop i4 (ref. 15); three intracellular loops i2, i3 and i4 mediate activation of the heterotrimeric G protein transducin; glutamic acid 135 and arginine 136 at the cytoplasmic end of helix III affect binding of transducin. But to provide a framework to interpret these data, not only for rhodopsin but for other G-protein-coupled receptors, requires the structure to be determined. Here we present a projection map of rhodopsin showing the configuration of the helices.

Amino-aromatic interaction between histidine 197 of the neurokinin-1 receptor and CP 96345

Substance P is a peptide neurotransmitter that binds to the neurokinin-1 receptor and is involved in pain transmission and neurogenic inflammation. Recently, a non-peptide substance P antagonist (CP 96345) has been shown to be effective in animal models of pain and inflammation. An understanding of the molecular interactions responsible for ligand binding will be critical for the development of more specific and selective antagonists for the neurokinin-1 receptor and for the discovery of new antagonists for related G-protein-coupled receptors. Here we report that histidine at position 197 in the fifth transmembrane helix of the human neurokinin-1 receptor binds specifically to CP 96345 but not to peptide agonists. Substitution of His 197 by different amino acids and analysis of structural analogues of antagonists suggest that His 197 is involved in an amino-aromatic interaction with the benzhydryl moiety of CP 96345.

Primary structure of frog rhodopsin

Amphibians have been employed extensively to study the anatomy, physiology, biochemistry, and cell biology of the visual system for decades, yet there have been no reports concerning the primary structure of amphibian visual transduction components. Thus, we have determined the entire nucleotide sequence of frog (Rana pipiens) rhodopsin cDNA, including a putative transcription start point and poly A tail, by sequence analysis of PCR products and mRNA. The open reading frame predicts an opsin of 354 residues, six residues longer than the mammalian rod opsins, containing 11 potential phosphorylation sites in the C-terminal domain. RNA blot analysis revealed two transcripts of ca. 1.7 and 3.1 kb. Frog rhodopsin exhibits approximately 85% identity to mammalian rhodopsin at the amino acid level. Sequence analysis of additional components will produce the framework from which a more detailed understanding of amphibian phototransduction can emerge.

Constitutively active mutants of rhodopsin

Two critical amino acids in the visual pigment rhodopsin are Lys-296, the site of attachment of retinal to the protein through a protonated Schiff base linkage, and Glu-113, the Schiff base counterion. Mutation of Lys-296 or Glu-113 results in constitutive activation of opsin, as assayed by its ability to activate transducin in the absence of added chromophore. We conclude that opsin is constrained to an inactive conformation by a salt bridge between Lys-296 and Glu-113. Recently, one of the mutants, K296E, was found in a family with retinitis pigmentosa, suggesting that degeneration of the photoreceptor cells in individuals with this mutation may result from persistent stimulation of the phototransduction pathway.

The structure of neuropeptide receptors

Recently, the primary structures of 17 different receptors for neuropeptides and small peptide hormones have been elucidated by molecular cloning. All but one belong to the superfamily of G-protein coupled receptors which share a topography consisting of seven transmembrane domains. Comparison of primary structures shows that two classes of peptide receptors exist. One referred to as the 'neurokinin-type receptors', possesses many of the typical, conserved amino acid sequence motifs of the aminergic transmitter receptors (e.g. beta-adrenoceptor). The other, referred to the 'secretin-type receptors', displays unrelated and distinctly different sequence motifs which are conserved between the three presently known members of this class. These are the secretin, calcitonin and parathyroid hormone/parathyroid hormone-like polypeptide receptors. One may speculate that many other peptides with a core of biological activity in the N-terminal or middle region may have receptors of the secretin-type.

Molecular determinants of spectral properties and signal transduction in the visual pigments

Site-directed mutagenesis of the visual pigment rhodopsin has provided a wealth of information regarding amino acid residues responsible for the determination of the spectral properties of the chromophore, the amino acids involved in activation and inactivation of the protein, and the effect of amino acid substitutions found in patients with retinitis pigmentosa. In addition, cell culture systems have now been established for expression of the three human color vision pigments, opening the way for a similar attack on the structure and function of these important proteins.