Alternative titles; symbols
HGNC Approved Gene Symbol: GNGT1
Cytogenetic location: 7q21.3 Genomic coordinates (GRCh38): 7:93,906,567-93,911,265 (from NCBI)
Heterotrimeric guanine nucleotide-binding proteins (G proteins) transduce extracellular signals received by transmembrane receptors to effector proteins. Transducin is a guanine nucleotide-binding protein found specifically in rod outer segments, where it mediates activation by rhodopsin of a cyclic GTP-specific (guanosine monophosphate) phosphodiesterase. Transducin is also referred to as GMPase. GNGT1 encodes the gamma subunit of transducin (Hurley et al., 1984; Scherer et al., 1996).
Hurley et al. (1984) isolated a cDNA clone for the gamma subunit of bovine transducin and Yatsunami et al. (1985) reviewed the amino acid sequence derived therefrom.
Scherer et al. (1996) reported the DNA sequence of GNGT1, the human transducin gamma subunit-1 gene. They screened a human genomic phage library using a human PCR product based on a partial human sequence published by Tao et al. (1993). One clone containing all of the coding sequence was identified, and the authors reported the full sequence of the coding region and its flanking intronic and noncoding regions. The putative open reading frame encodes a 74-amino acid polypeptide.
In heterotrimeric, alpha/beta/gamma G proteins, the alpha subunit (e.g., 139330) shows great diversity and has been thought to confer functional specificity to a particular G protein. By contrast, the beta (e.g., 139380) and gamma subunits are less diverse and have been thought to play no role in G protein specificity. However, using immunocytochemistry, Peng et al. (1992) found distinct distribution patterns for different beta and gamma subunits in the retina. In particular, rod and cone photoreceptors, which both subserve phototransduction but differ in light-response properties, had different beta and gamma subunits in their outer segments. Thus, the G protein that mediates phototransduction shows cell-specific forms of the beta and gamma subunits in addition to the alpha subunit.
G proteins are posttranslationally modified by either farnesylation or geranylgeranylation. The gamma subunit of retinal transducin is farnesylated, and this modification is required for light signaling in rod cells. Kassai et al. (2005) found that knockin mice expressing geranylgeranylated transducin-gamma showed normal rod responses to dim flashes under dark adaptation, but showed impaired properties in light adaptation. Geranylgeranylation also attenuated light-dependent translocation of transducin from the outer segment to the inner region. Kassai et al. (2005) concluded that farnesylation and geranylgeranylation of transducin-gamma are interchangeable in terms of light signaling, but specific farnesylation is important for visual sensitivity regulation by providing sufficient but not excessive membrane anchoring of the transducin beta/gamma subunits.
Scherer et al. (1996) determined that the GNGT1 gene contains 3 exons and 2 introns.
Scherer et al. (1996) used a panel of somatic cell hybrids and a collection of chromosome 7-specific YACs to localize the GNGT1 gene to 7q21.3. They noted that transducin and other genes encoding phototransduction products are considered to be candidates for retinitis pigmentosa (RP) and other retinal defects.
Each subunit of the G protein complex is encoded by a member of 1 of 3 corresponding gene families. Hurowitz et al. (2000) counted 16 different members of the alpha-subunit family, 5 different members of the beta-subunit family, and 11 different members of the gamma-subunit family, as described in mammals. They identified and characterized BACs containing the human homologs of each of the alpha-, beta-, and gamma-subunit genes. The gene structure and chromosome location of each gene was determined, as were the orientations of paired genes.
Hurley, J. B., Fong, H. K. W., Teplow, D. B., Dreyer, W. J., Simon, M. I. Isolation and characterization of a cDNA clone for the gamma subunit of bovine retinal transducin. Proc. Nat. Acad. Sci. 81: 6948-6952, 1984. [PubMed: 6438626] [Full Text: https://doi.org/10.1073/pnas.81.22.6948]
Hurowitz, E. H., Melnyk, J. M., Chen, Y.-J., Kouros-Mehr, H., Simon, M. I., Shizuya, H. Genomic characterization of the human heterotrimeric G protein alpha, beta, and gamma subunit genes. DNA Res. 7: 111-120, 2000. [PubMed: 10819326] [Full Text: https://doi.org/10.1093/dnares/7.2.111]
Kassai, H., Aiba, A., Nakao, K., Nakamura, K., Katsuki, M., Xiong, W.-H., Yau, K.-W., Imai, H., Shichida, Y., Satomi, Y., Takao, T., Okano, T., Fukada, Y. Farnesylation of retinal transducin underlies its translocation during light adaptation. Neuron 47: 529-539, 2005. [PubMed: 16102536] [Full Text: https://doi.org/10.1016/j.neuron.2005.07.025]
Peng, Y.-W., Robishaw, J. D., Levine, M. A., Yau, K.-W. Retinal rods and cones have distinct G protein beta and gamma subunits. Proc. Nat. Acad. Sci. 89: 10882-10886, 1992. [PubMed: 1438293] [Full Text: https://doi.org/10.1073/pnas.89.22.10882]
Scherer, S. W., Feinstein, D. S., Oliveira, L., Tsui, L.-C., Pittler, S. J. Gene structure and chromosome localization to 7q21.3 of the human rod photoreceptor transducin gamma-subunit gene (GNGT1). Genomics 35: 241-243, 1996. [PubMed: 8661128] [Full Text: https://doi.org/10.1006/geno.1996.0346]
Tao, L., Pandey, S., Simon, M. I., Fong, H. K. W. Structure of the bovine transducin gamma subunit gene and analysis of promoter function in transgenic mice. Exp. Eye Res. 56: 497-507, 1993. [PubMed: 8500562] [Full Text: https://doi.org/10.1006/exer.1993.1063]
Yatsunami, K., Pandya, B. V., Oprian, D. D., Khorana, H. G. cDNA-derived amino acid sequence of the gamma subunit of GTPase from bovine rod outer segments. Proc. Nat. Acad. Sci. 82: 1936-1940, 1985. [PubMed: 2984674] [Full Text: https://doi.org/10.1073/pnas.82.7.1936]