Anion sensitivity and spectral tuning of cone visual pigments in situ.

Abstract

We tested the effect of anions on the absorbance spectrum of native visual pigments as measured by microspectrophotometry in individual cone outer segments of four species of fish and one species of amphibian. In all species tested, the long-wavelength-absorbing cone pigments were anion sensitive, and their A.. could be tuned over a range of 55 nm depending on the identity of the anion present. Cl-and Br-were the only anions that produced native pigment spectra by red shifting A.. from its value under anion-free conditions. Lyotropic anions such as NO-, SCN-, BF4, and ClIO caused substantial and graded blue shifts of A... The apparent Kd of binding sites on the pigment for Cl-and for CIO4 was -2 mM. Taken together with previous findings on three visual pigments from the reptilian, avian, and amphibian classes, our results support the hypothesis that all long-wavelength-absorbing vertebrate visual pigments are spectrally tuned in part through the binding of a chloride ion. We propose that the site of anion tuning is near the protonated Schiff base of the chromophore, whose counterion may be complex and include Cl-as an exchangeable anion. This counterion configuration may resemble the one present in the light-driven Cl-pump halorhodopsin.Color vision in vertebrates is based on sets of two, three, or four different photopigments that reside in separate classes of cone photoreceptors. The wavelength of peak absorbance (Amax) of these pigments ranges from the UV (360 nm) to the far red (635 nm). All vertebrate visual pigments are integral membrane proteins and contain 11-cis-retinal or 11-cisdehydroretinal as the chromophore that is covalently bound through a Schiffbase linkage to a lysine residue on the protein (opsin) moiety ofthe pigment (1). Although great progress has been made in recent years in unraveling the structure of visual pigments, the molecular basis of spectral tuning is still only incompletely understood.A variety of mechanisms have been invoked to account for photopigment tuning (for reviews, see refs. 2 and 3). For example, protonation of the retinal Schiff base shifts Amax from 360 to 430 nm (4). Blue-absorbing pigments may contain an unperturbed chromophore behaving much like a protonated retinal Schiff base in a nonpolar solvent (5). Closer interaction of the chromophore with the protein perturbs the electronic structure of the chromophore and results in an additional red shift of Am,,,. This shift is commonly called the "opsin shift" and very likely comprises multiple components. For example, Amax is extremely sensitive to the charge environment provided by the counterion, which is paired with the protonated Schiff base (3). A decreased interaction between protonated Schiff base and counterion, perhaps caused by an increased distance between the two, may be responsible for the opsin shift in green-absorbing rhodopsins (Amax, "500 nm). Amino acid side chains, which form the hydrophobic binding pocket for the chromophore, also can interact with and perturb the chromophore. In fact, much ...

Into the blue: Gene duplication and loss underlie color vision adaptations in a deep-sea chimaera, the elephant sharkCallorhinchus milii

“…Many LWS pigments are anion sensitive (Kleinschmidt and Harosi, 1992), with His181 and Lys184 forming a chloride-binding site (Wang et al 1993). Replacement of His181, as has occurred naturally in the mouse LWS pigment, abolishes chloride binding and generates a 28-nm short wavelength shift in l max (Sun et al 1997).…”

“…1% (w/v) dodecyl-maltoside and 20 mg/ mL phenylmethanesulphonylfluoride (PMSF) was then added before passage over a CNBr-activated sepharose-binding column coupled to an anti-1D4 monoclonal antibody (Molday and MacKenzie 1983). In chloride-free experiments, all buffers used in the regeneration of elephant shark LWS1 and LWS2 visual pigments comprised of 13 PBS (pH 7.0), where 2.7 mM KCl and 137 mM NaCl were replaced with 2.7 mM KOH and 137 mM NaClO 4 , respectively, as according to Kleinschmidt and Harosi (1992).…”

Evolution and spectral tuning of visual pigments in birds and mammals

“…The LWS pigment in the mouse is blue-shifted to 508 nm, as a result of the loss of the chloride-binding site. Many LWS pigments are anion-sensitive (Kleinschmidt & Harosi 1992), with His181 and Lys184 forming a chloride-binding site (Wang et al 1993). Replacement of His181, which has occurred naturally in the mouse LWS pigment, abolishes chloride binding and generates a 28 nm short wavelength shift in l max (Sun et al 1997).…”

A Photochromic Histidine Kinase Rhodopsin (HKR1) That Is Bimodally Switched by Ultraviolet and Blue Light

“…The respective spectral sensitivity of rhodopsins may cover an extremely broad range of the sunlight spectrum, from 358 nm (near ultraviolet, UVA) to 630 nm (near infrared) (1,2). Rhodopsins with absorption at the red edge of the spectrum (XL-iodopsins) are found in flies, mollusks, fish, and in some birds.…”