Amino acid sequence of the alpha subunit of transducin deduced from the cDNA sequence.

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Abstract
Transducin, a GTP-binding protein involved in phototransduction in the vertebrate retina, belongs to a family of homologous coupling proteins that also includes Gs and Gi, the regulatory proteins of adenylate cyclase. The c[complementary]DNA sequence and deduced amino acid sequence of transducin''s .alpha. subunit (T.alpha.) are reported. The cDNA was isolated, by screening with an antibody probe, from a bovine retinal cDNA library in the expression vector .lambda.gt11. The 2.2-kilobase cDNA insert hybridized to a single 2.6-kilobase poly(A)+ RNA species present in extracts of bovine retina but not of bovine heart, liver, or brain. The nucleotide sequence of the cDNA revealed an open reading frame long enough to encode the entire 39-kDa [kilodalton] T.alpha. polypeptide. The polypeptide sequence deduced from the cDNA would be composed of 350 amino acids and have a MW of 39,971. Portions of the sequence matched reported amino acid sequences of T.alpha. tryptic fragments, including sites specifically ADP-ribosylated by cholera and pertussis toxins. The predicted sequence also includes 4 segments, ranging from 11-19 residues in length, that exhibit significant homology to sequences of GTP-binding proteins, including the ras proteins of man and yeast and the elongation factors of ribosomal protein synthesis in bacteria, EF-G and EF-Tu. In combination with previous functional studies of tryptic fragments of T.alpha., the deduced amino acid sequence makes it possible to predict which portions of the polypeptide interact with other molecules involved in retinal phototransduction.