Primary structure of the α-subunit of transducin and its relationship to ras proteins

Abstract

A group of membrane-associated guanine nucleotide binding proteins (G-proteins) are essential for transducing signals generated at cell-surface receptors into changes in cellular function and metabolism. These proteins are a complex of three subunits designated alpha, beta and gamma. The alpha-subunit is responsible for binding guanine nucleotides and seems to be characteristic of each protein. Transducin, a member of this protein family, mediates visual transduction by coupling the signal of photolysed rhodopsin with activation of a cyclic GMP phosphodiesterase. We have now cloned and sequenced the complementary DNA encoding the alpha-subunit of bovine retinal transducin and from this we have deduced the complete amino-acid sequence. The transducin alpha-subunit shares several homologous amino-acid sequences with ras gene products. The homologous segments correspond mostly to the regions thought to be involved in the guanine nucleotide binding and GTPase activity of ras proteins and to the ADP-ribosylation sites of the transducin alpha-subunit.

Why photoreceptors die (and why they don't)

“…(18)(19)(20) As in other such cascades, the binding of the G protein to the receptor causes the exchange of GTP for GDP on the guanylyl nucleotide binding site of the transducin a subunit (T a ). The T a -GTP then separates from the T b and T g subunits of the transducin and binds to the effector enzyme of the cascade, a photoreceptor-specific cyclic nucleotide phosphodiesterase (PDE).…”

Characterization of four GO‐type proteins purified from bovine brain membranes

“…The cr subunits of Gi serve as the substrate of pertussis toxin-(lAP-) catalyzed ADP-ribosylation in their trimeric forms, have been classified into three subtypes of Oti-1, O/i-2 and o'i-3 from analyses of molecular cloning of the genes [2] and cDNAs [3][4][5][6]. In regard to the cesubunits of Go, at least two types of the genes were informed from Northern blot analysis [7].…”

A structural model for the α‐subunit of transducin Implications of its role as a molecular switch in the visual signal transduction mechanism

“…Biochemical data on the structural and functional relationships of the T, molecule are now available and include the primary sequence elucidated via molecular cloning [4-71, peptide mapping [8], and functional domain analysis through chemical modification [9,10]. Moreover, the homology between the GTP-binding site of T, and those of the G-proteins coupled to adenylate cyclase, elongation factor Tu (EF-Tu) and the ras p21 protein has been identified [4][5][6][7]111. Recently, the tertiary structure of the GTP-binding domain of EF-Tu was solved to a resolution of 2.7 A by Xray crystallography [12,13] and has served as a model for the ras gene product [14] and for the partial folding of a consensus GTP-binding site [15].…”