MII catalyses GDP/GTP exchange on Gt

Stable Identifier
R-HSA-2485180
Type
Reaction [transition]
Species
Homo sapiens
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In darkness, the G protein transducin (Gt) is attached to the disk membrane surface with a GDP bound to it and it is inactive. Gt is a heterotrimer of alpha1 (GNAT1) (van Dop et al. 1989, Fong 1992), beta1 (GNB1) (Codina et al. 1986) and gamma1 (GNGT1) (Tao et al. 1993) subunits. Photoactivated rhodopsin (MII or R*) catalyzes the exchange of GTP for GDP bound to Gt. Upon GTP/GDP exchange, Gt is released from MII and the Gt alpha with GTP bound (GNAT1 GTP) dissociates from Gt beta gamma subunits (GNB1:GNGT1). This mechanism was deciphered from bovine experiments (Pugh & Lamb 1993). MII proceeds to activate additional Gt molecules, making this reaction the first amplification step in the phototransduction cascade. A single activated rhodopsin molecule activates tens of Gt molecules. Although phosphorylation of activated rhodopsin (MII) by rhodopsin kinase (GRK1) reduces transducin activation (Khani et al. 1996), complete deactivation occurs only after arrestin (S-antigen or SAG, Yamaki et al. 1988) binds to and sterically caps MII.

Defects in GNAT1 cause the Nougaret type of autosomal dominant, congenital stationary night blindness (Dryja et al. 1996, CSNBAD3; MIM:610444) . Congenital stationary night blindness is a non progressive retinal disorder characterized by impaired night vision.

Literature References
PubMed ID Title Journal Year
8382952 Amplification and kinetics of the activation steps in phototransduction

Pugh, EN, Lamb, TD

Biochim. Biophys. Acta 1993
1614872 Characterization of the human rod transducin alpha-subunit gene

Fong, SL

Nucleic Acids Res. 1992
8673138 Missense mutation in the gene encoding the alpha subunit of rod transducin in the Nougaret form of congenital stationary night blindness

Dryja, TP, Hahn, LB, Reboul, T, Arnaud, B

Nat Genet 1996
3095147 Beta-subunits of the human liver Gs/Gi signal-transducing proteins and those of bovine retinal rod cell transducin are identical

Codina, J, Stengel, D, Woo, SL, Birnbaumer, L

FEBS Lett 1986
2748346 Nucleotide sequence for a cDNA encoding the alpha subunit of retinal transducin (GNAT1) isolated from the human eye

Van Dop, C, Medynski, DC, Apone, LM

Nucleic Acids Res 1989
8812493 Characterization and chromosomal localization of the gene for human rhodopsin kinase

Khani, SC, Abitbol, M, Yamamoto, S, Maravic-Magovcevic, I, Dryja, TP

Genomics 1996
3164688 The sequence of human retinal S-antigen reveals similarities with alpha-transducin.

Yamaki, K, Tsuda, M, Shinohara, T

FEBS Lett 1988
8500562 Structure of the bovine transducin gamma subunit gene and analysis of promoter function in transgenic mice

Tao, L, Pandey, S, Simon, MI, Fong, HK

Exp. Eye Res. 1993
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Catalyst Activity
Catalyst Activity
Title
guanyl-nucleotide exchange factor activity of at-retinyl-RHO [photoreceptor disc membrane]
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