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The Gal4 transcription factor is a positive regulator of gene expression of galactose-induced genes.[1] This protein represents a large fungal family of transcription factors, Gal4 family, which includes over 50 members in the yeast Saccharomyces cerevisiae e.g. Oaf1, Pip2, Pdr1, Pdr3, Leu3.[2]
Two executive domains, DNA binding and activation domains, provide key function of the Gal4 protein conforming to most of the transcription factors.
DNA binding
Gal4 N-terminus is a zinc finger and belongs to the Zn(2)-C6 fungal family. It forms a Zn – cysteines thiolate cluster,[5][6] and specifically recognizes UASG in GAL1 promoter. [7][8]
Gal4 transactivation
Localised to the C-terminus, belongs to the nine amino acids transactivation domain family, 9aaTAD, together with Oaf1, Pip2, Pdr1, Pdr3, but also p53, E2A, MLL.[9][10]
Regulation
Galactose induces Gal4 mediated transcription albeit Glucose causes severe repression.[11][12]
As a part of the Gal4 regulation, inhibitory protein Gal80 recognises and binds to the Gal4 region (853-874 aa).[13][14][15]
The inhibitory protein Gal80 is sequestered by regulatory protein Gal3 in Galactose dependent manner. This allows for Gal4 to work when there is galactose.[16][4][17][18]
Mutants
The Gal4 loss-of-function mutant gal4-64 (1-852 aa, deletion of the Gal4 C-terminal 29 aa) lost both interaction with Gal80 and activation function.[19][20][21]
In the Gal4 reverted mutant Gal4C-62 mutant,[22] a sequence (QTAY N AFMN) with the 9aaTAD pattern emerged and restored activation function of the Gal4 protein.
Inactive constructs
The activation domain Gal4 is inhibited by C-terminal domain in some Gal4 constructs.[23][24]
The Gal4 protein interacts also with other mediators of transcription as are Tra1,[32][33][34] TAF9,[35] and SAGA/MED15 complex.[36][37]
Proteosome
A subunit of the 26 S proteasome Sug2 regulatory protein has a molecular and functional interaction with Gal4 function.[38][39] Proteolytic turnover of the Gal4 transcription factor is not required for function in vivo.[40] The native Gal4 monoubiquitination protects from 19S-mediated destabilizing under inducing conditions.[41]
Application
The broad use of the Gal4 is in yeast two-hybrid screening to screen or to assay protein-protein interactions in eukaryotic cells from yeast to human.
In the GAL4/UAS system, the Gal4 protein and Gal4 upstream activating region (UAS) are used to study the gene expression and function in organisms such as the fruit fly.[3]
The Gal4 and inhibitory protein Gal80 have found application in a genetics technique for creating individually labeled homozygous cells called MARCM (Mosaic analysis with a repressible cell marker).
^Klar AJ, Halvorson HO (1974). "Studies on the positive regulatory gene, GAL4, in regulation of galactose catabolic enzymes in Saccharomyces cerevisiae". Molecular & General Genetics. 135 (3): 203–12. doi:10.1007/BF00268616. PMID4376212. S2CID26014344.
^Pan T, Coleman JE (March 1990). "The DNA binding domain of GAL4 forms a binuclear metal ion complex". Biochemistry. 29 (12): 2023–9. doi:10.1021/bi00464a019. PMID2186803.
^Klar AJ, Halvorson HO (1974). "Studies on the positive regulatory gene, GAL4, in regulation of galactose catabolic enzymes in Saccharomyces cerevisiae". Molecular & General Genetics. 135 (3): 203–12. doi:10.1007/BF00268616. PMID4376212. S2CID26014344.
^Johnston SA, Salmeron JM, Dincher SS (July 1987). "Interaction of positive and negative regulatory proteins in the galactose regulon of yeast". Cell. 50 (1): 143–6. doi:10.1016/0092-8674(87)90671-4. PMID3297350. S2CID46090047.
^Johnston SA, Salmeron JM, Dincher SS (July 1987). "Interaction of positive and negative regulatory proteins in the galactose regulon of yeast". Cell. 50 (1): 143–6. doi:10.1016/0092-8674(87)90671-4. PMID3297350. S2CID46090047.
^Hashimoto H, Kikuchi Y, Nogi Y, Fukasawa T (1983). "Regulation of expression of the galactose gene cluster in Saccharomyces cerevisiae. Isolation and characterization of the regulatory gene GAL4". Molecular & General Genetics. 191 (1): 31–8. doi:10.1007/BF00330886. PMID6350827. S2CID115216273.
^Nogi Y, Fukasawa T (October 1980). "A novel mutation that affects utilization of galactose in Saccharomyces cerevisiae". Current Genetics. 2 (2): 115–20. doi:10.1007/BF00420623. PMID24189802. S2CID12635991.