Farnesylated γ-subunit of photoreceptor G protein indispensable for GTP-binding

Abstract

TRANSDUCIN, composed of subunits Tα, Tβ and Tγ, is a member of a heterotrimeric G-protein family, and transduces the light signal in visual cells. We have recently found that bovine Tβγ can be separated into two components, Tβγ-1 and Tβγ-2, each of which has its own γ-subunit, Tγ-1 and Tγ-2, respectively¹. Tβγ-2 enhances the binding of GTP to Tα in the presence of metarhodopsin II by about 30-fold compared with Tβγ-1 (ref. 1). Here we show that a farnesyl moiety is attached to a sulphur atom of the C-terminal cysteine of Tγ-2 (active form), a part of which is additionally methyl-esterified at the α-carboxyl group. In Tγ-1 (inactive form), however, such modifications are missing. Thus, the farnesyl moiety attached to the γ-subunit is indispensable for the GTP-binding activity of transducin. This suggests that a similar modification may occur in the γ-subunits of other heterotrimeric G proteins involved in biological signal transduction processes.