A Conserved Aromatic Residue Regulating Photosensitivity in Short-Wavelength Sensitive Cone Visual Pigments

Abstract

Visual pigments have a conserved phenylalanine in transmembrane helix 5 located near the β-ionone ring of the retinal chromophore. Site-directed mutants of this residue (F207) in a short-wavelength sensitive visual pigment (VCOP) were studied using UV-visible spectroscopy to investigate its role in photosensitivity and formation of the light-activated state. The side chain is important for pigment formation: VCOP(F207A), VCOP(F207L), VCOP(F207M), and VCOP(F207W) substitutions all bound 11-cis-retinal and formed a stable visual pigment, while VCOP(F207V), VCOP(F207S), VCOP(F207T), and VCOP(F207Y) substitutions do not. The extinction coefficients of all pigments are close, ranging between 35800 and 45600 M⁻¹ cm⁻¹. Remarkably, the mutants exhibit an up to 5-fold reduction in photosensitivity and also abnormal photobleaching behavior. One mutant, VCOP(F207A), forms an isomeric composition of the retinal chromophore after illumination comparable to that of wild-type VCOP yet does not release the all-trans-retinal chromophore. These findings suggest that the conserved F207 residue is important for a normal photoactivation pathway, formation of the active conformation and the exit of all-trans-retinal from the chromophore-binding pocket.

Spectral tuning in vertebrate short wavelength‐sensitive 1 (SWS1) visual pigments: Can wavelength sensitivity be inferred from sequence data?

“…Though we focus on spectral tuning mechanisms in this review, UVS and VS SWS1 pigments also differ in other physicochemical characteristics aside from absorption maxima, and these differences are likely influenced by the protonation state of the Schiff base. It should be noted, however, that an aromatic residue near the b-ionone ring of the chromophore in SWS1 pigments has also been shown to mediate non-spectral properties in these pigments (Kuemmel et al, 2013). UVS SWS1 pigments have been found to possess a slower retinal release (Chen et al, 2012), a more tightly packed binding pocket (Das et al, 2004), increased dark state stability (Luo et al, 2011), and a narrower absorption curve bandwidth relative to VS pigments (Govardovskii et al, 2000;Tsutsui and Shichida, 2010).…”

Advances in understanding the molecular basis of the first steps in color vision

“…For instance, conserved in human cone pigments, the F207 residue plays an important role in photoactivation and a minor role in spectral tuning. Replacing F207 with a V, Y, T or S consistently leads to a reduction or absence of absorption without changing λ max (Kuemmel et al, 2013). Moreover, a less conserved acidic residue in visual pigments in ECL2 D181 or E181 plays a crucial role in GPCR activation as a counter ion switch (Terakita et al, 2004, 2000; Yan et al, 2003).…”