Sequence of the Alpha Subunit of Photoreceptor G Protein: Homologies between Transducin, ras, and Elongation Factors

A bovine retinal complementary DNA clone encoding the α subunit of transducin (T_α) was isolated with the use of synthetic oligodeoxynucleotides as probes, and the complete nucleotide sequence of the insert was determined. The predicted protein sequence of 354 amino acids includes the known sequences of four tryptic peptides and sequences adjacent to the residues that undergo adenosine diphosphate ribosylation by cholera toxin and pertussis toxin. On the basis of homologies to other proteins, such as the elongation factors of protein synthesis and the ras oncogene proteins, regions are identified that are predicted to be acylated and involved in guanine nucleotide binding and hydrolysis. Amino acid sequence similarity between T_α and ras is confined to these regions of the molecules.