Histidine residues regulate the transition of photoexcited rhodopsin to its active conformation, metarhodopsin II.
The biologically active photoproduct of rhodopsin, metarhodopsin II (M II), exists in a pH-sensitive equilibrium with its precursor, metarhodopsin I (M I). Increasing acidity favors M II, with the midpoint of the pH titration curve at pH 6.4. To test the long-standing proposal that histidine protonation regulates this conformational transition, we characterized mutant rhodopsins in which each of the 6 histidines was replaced by phenylalanine or cysteine. Only mutants substituted at the 3 conserved histidines showed abnormal M I-M II equilibria. Those in which His-211 was replaced by phenylalanine or cysteine formed little or no M II at either extreme of pH, whereas mutants substituted at His-65 or at His-152 showed enhanced sensitivity to protons. The simplest interpretation of these results is that His-211 is the site where protonation strongly stabilizes the M II conformation and that His-65 and His-152 are sites where protonation modestly destabilizes the M II conformation.
Weitz
C J
CJ
Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Nathans
J
J
D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D004252:DNA Mutational Analysis; D004072:Digitonin; D005136:Eye Proteins / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D010777:Photochemistry; D011994:Recombinant Proteins / Q000737:chemistry; D012243:Rhodopsin / Q000031:analogs & derivatives* / Q000737:chemistry*; D017299:Rod Opsins; D012995:Solubility; D013057:Spectrum Analysis
D005136:Eye Proteins; D010455:Peptides; D011994:Recombinant Proteins; D017299:Rod Opsins; D006639:Histidine; C013110:metarhodopsins; D012243:Rhodopsin; D004072:Digitonin