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A group of effector signalling proteins, associated with G coupled signalling. The most well characterised are the visual pigments such as rhodopsin but other members of the group are expressed in diverse tissues.


Visual Opsins

Vertebral visual pigments are seven-transmembrane segment proteins covalently linked to the chromophore, 11-cis retinal.


The basic vertebral system appears to have evolved from a five cone opsin base[1] In general in higher vertebrates there is:

  • An opsin with an absorption maximum at <500 nm
  • An opsin with an absorption maximum at >500 nm.
  • Rhodopsin with an absorption maximum at about 500 nm and plays little or no role in color vision but is used under dim light conditions.

Humans have four visual pigments[2]:

  • A single member of the <500 nm family of cone pigments (Opsin 1 short-wave-sensitive - absorption maximum at 425 nm)
  • Two highly homologous members of the >500 nm family coded for on the X chromosome
    • Result of gene duplication in the Old World primate lineage at about 30–40 million years ago
      • Could have resulted from:
        1. Initially identical genes that subsequently diverged
        2. Cross over of two X chromosomes that carried polymorphic variants with different absorption spectra
    • Reside in head-to-tail tandem array on the X chromosome and show 98% DNA sequence identity in the coding, intron, and 3′ flanking sequences
    1. Opsin 1 medium-wave-sensitive ( - absorption maximum at 530 nm)
    2. Opsin 1 long-wave-sensitive (absorption maxima at 560 nm)
  • Rhodopsin

Evolutionary proof

It has proved possible to genetically engineer mice (which normally see in two colours) by adding the human gene for red sensitive opsin and these mice can then discriminate three colours due to the brains plasticity[3]


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