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Schiff-base deprotonation is mandatory for light-dependent rhodopsin phosphorylation.

ABSTRACT: The absorption of light by rhodopsin leads to the formation of an activated intermediate (R*) capable of catalysing the exchange of GTP for GDP in a retinal guanine-nucleotide-binding regulatory protein (transducin). The ability of R* to function as a catalyst is terminated by the rhodopsin kinase. The 10 nonactive-site lysine residues of rhodopsin can be reductively dimethylated to form permethylated rhodopsin (PMRh). This derivative is phosphorylated to the same extent as rhodopsin after photolysis. The monomethylation of the active-site lysine residue of PMRh yields active-site-methylated rhodopsin (AMRh). It had previously been shown, by using AMRh, that the formation of R* and its spectroscopic signature metarhodopsin II requires the photochemically induced deprotonation of the active-site Schiff base [Longstaff, Calhoon & Rando (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 4209-4213]. Here it is demonstrated that active-site Schiff-base deprotonation is also mandatory in the formation of the form of photolyzed rhodopsin that is susceptible to phosphorylation by rhodopsin kinase. In terms of the spectroscopically defined rhodopsin intermediates, this means that only metarhodopsin II and possibly metarhodopsin III are the actual substrates for rhodopsin kinase.

SUBMITTER: Seckler B

PROVIDER: S-EPMC1133606 | biostudies-other | 1989 Dec

REPOSITORIES: biostudies-other

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Project description:One of the distinctive features of eubacterial retinal-based proton pumps, proteorhodopsins, xanthorhodopsin, and others, is hydrogen bonding of the key aspartate residue, the counterion to the retinal Schiff base, to a histidine. We describe properties of the recently found eubacterium proton pump from Exiguobacterium sibiricum (named ESR) expressed in Escherichia coli, especially features that depend on Asp-His interaction, the protonation state of the key aspartate, Asp85, and its ability to accept a proton from the Schiff base during the photocycle. Proton pumping by liposomes and E. coli cells containing ESR occurs in a broad pH range above pH 4.5. Large light-induced pH changes indicate that ESR is a potent proton pump. Replacement of His57 with methionine or asparagine strongly affects the pH-dependent properties of ESR. In the H57M mutant, a dramatic decrease in the quantum yield of chromophore fluorescence emission and a 45 nm blue shift of the absorption maximum with an increase in the pH from 5 to 8 indicate deprotonation of the counterion with a pK(a) of 6.3, which is also the pK(a) at which the M intermediate is observed in the photocycle of the protein solubilized in detergent [dodecyl maltoside (DDM)]. This is in contrast with the case for the wild-type protein, for which the same experiments show that the major fraction of Asp85 is deprotonated at pH >3 and that it protonates only at low pH, with a pK(a) of 2.3. The M intermediate in the wild-type photocycle accumulates only at high pH, with an apparent pK(a) of 9, via deprotonation of a residue interacting with Asp85, presumably His57. In liposomes reconstituted with ESR, the pK(a) values for M formation and spectral shifts are 2-3 pH units lower than in DDM. The distinctively different pH dependencies of the protonation of Asp85 and the accumulation of the M intermediate in the wild-type protein versus the H57M mutant indicate that there is strong Asp-His interaction, which substantially lowers the pK(a) of Asp85 by stabilizing its deprotonated state.

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Schiff-base deprotonation is mandatory for light-dependent rhodopsin phosphorylation.

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