No abstract

The chicken retina contains rhodopsin (a rod visual pigment) and four kinds of cone visual pigments. The

In avian pinealocytes, an environmental light signal resets the phase of the endogenous circadian pacemaker that controls the rhythmic production of melatonin. Investigation of the pineal phototransduction pathway should therefore reveal the molecular mechanism of the biological clock. The presence of rhodopsin-like photoreceptive pigment, transducin-like immunoreaction, and cyclic GMP-dependent cation-channel activity in the avian pinealocytes suggests that there is a similarity between retinal rod cells and pinealocytes in the phototransduction pathway. We have now cloned chicken pineal cDNA encoding the photoreceptive molecule, which is 43-48% identical in amino-acid sequence to vertebrate retinal opsins. Pineal opsin, produced by transfection of complementary DNA into cultured cells, was reconstituted with 11-cis-retinal, resulting in formation of a blue-sensitive pigment (lambda max approximately 470 nm). In the light of this functional evidence and because the gene is specifically expressed only in the pineal gland, we conclude that it is a pineal photosensor and name it pinopsin.

Transducin, composed of subunits T alpha, T beta and T gamma, is a member of a heterotrimeric G-protein family, and transduces the light signal in visual cells. We have recently found that bovine T beta gamma can be separated into two components. T beta gamma-1 and T beta gamma-2, each of which has its own gamma-subunit, T gamma-1 and T gamma-2, respectively. T beta gamma-2 enhances the binding of GTP to T alpha in the presence of metarhodopsin II by about 30-fold compared with T beta gamma-1. Here we show that a farnesyl moiety is attached to a sulphur atom of the C-terminal cysteine of T gamma-2 (active form), a part of which is additionally methyl-esterified at the alpha-carboxyl group. In T gamma-1 (inactive form), however, such modifications are missing. Thus, the farnesyl moiety attached to the gamma-subunit is indispensable for the GTP-binding activity of transducin. This suggests that a similar modification may occur in the gamma-subunits of other heterotrimeric G proteins involved in biological signal transduction processes.