Structure and function of gamma-subunit of photoreceptor G-protein (transducin)

1. The gamma-subunit of the vertebrate photoreceptor GTP-binding protein (transducin) is S-farnesylated at the C-terminal cysteine residue, with a part of the residue being methyl-esterified at the alpha-carboxyl group. 2. Functionally, the modified cysteine residue is implicated in efficient coupling of the alpha- and beta gamma-subunits, and indispensible for expressing GTP-binding activity. 3. Similar modifications, isoprenylation and methyl-esterification of the C-terminal cysteine residue have been found in a variety of proteins involved in signal transduction and growth regulation processes. However, it seems likely that the physiological roles of the modifications are different for the various proteins.