酵母雙雜交系統 Protein–protein interactions occur in many biological processes including replication, transcription, secretion, signal transduction, and metabolism. A fundamental question in the study of any protein is to identify proteins that interact with a given protein in vivo. Intense research efforts are focused on the identification of these proteins.
The GAL4 two-hybrid phagemid vector system, a eukaryotic
system to detect protein–protein interactions in vivo, provides a method for the rapid identification of genes encoding proteins that interact with a given protein (i.e., a bait protein).1,2 The system is based on the ability to separate eukaryotic transcriptional activators into two separate domains, the DNAbinding domain (BD) and the transcriptional activation domain (AD).
 In theGAL4 two-hybrid phagemid vector system, proteins that interact with the bait protein are identified by generating hybrids of the yeast GAL4 BD and the bait protein (X) and the GAL4 AD and a library of proteins (Y). Neither hybrid protein is capable of initiating specific transcription of reporter genes in yeast in the absence of a specific interaction with the other hybrid protein. 
When the hybrid protein X is expressed in yeast, the GAL4 BD binds X to specific DNA sequences in the yeast chromosome defined by the GAL1 or GAL4 upstream activating sequences (UASGAL1 or UASGAL4, respectively), which regulate the expression of a reporter gene. Binding of X to the UAS is not sufficient to initiate transcription of the reporter gene. When Y is expressed in yeast, the AD interacts with other components of the transcription machinery required to initiate transcription of the reporter gene. However, Y alone is not localized to the reporter gene UAS and therefore does not activate transcription of the reporter gene. When a specific interaction between X and Y localizes both the GAL4 BD and GAL4 AD to the reporter gene UAS, transcriptional activation of the reporter gene occurs (Figure 2B). The reporter genes in the GAL4 twohybrid phagemid vector system are β-galactosidase (lacZ) and histidine (HIS3). 雙雜交載體pAD-GAL4-2.1和pBD-GAL4-Cam The pAD-GAL4-2.1 phagemid vector contains a multiple cloning site (MCS) with BamH I, Nhe I, EcoR I, Xho I, Sal I, Xba I, Pst I, and Bgl II restriction sites. The pBD-GAL4-Cam phagemid vector contains an MCS with EcoR I, Srf I, Sma I, Xho I, Sal I, Xba I, and Pst I restriction sites. The unique EcoR I and Xho I cloning sites in the pAD-GAL4-2.1 vector make this vector compatible with the Agilent cDNA Synthesis Kit for the preparation of unidirectional cDNA libraries.
The unique EcoR I and Sal I cloning sites are used for the preparation of cDNA libraries in the pBD-GAL4 Cam phagemid vector because the Xho I site in the MCS is not unique. The unique BamH I, Nhe I, and EcoR I sites at the 5′ end and the Xho I, Sal I, Xba I, and Bgl II sites at the 3′ end of the DNA insert facilitate the transfer of DNA encoding the target protein into commonly used protein expression/purification vectors. Genomic DNA digested with Mbo I, BamH I, or Sau3A I and partially filled-in can be inserted into a partially filled-in Xho I site in the pAD-GAL4-2.1 phagemid vector. The Xba I site in the pAD-GAL4-2.1 and pBD-GAL4-Cam phagemid vectors is not unique and contains the UAG amber suppressor in the same translational reading frame as the GAL4 domain. DNA should therefore be inserted such that the Xba I site is not between the GAL4 domain and the DNA insert.
The pAD-GAL4-2.1 and pBD-GAL4 Cam phagemid vectors contain the pUC origin for replication and an f1 origin for production of single-stranded DNA (ssDNA) in E. coli. Single-stranded DNA can be used for DNA sequencing or site-directed mutagenesis. The pAD-GAL4-2.1 and pBDGAL4-Cam phagemid vectors contain ampicillin-resistance gene [β-lactamase (bla)] and chloramphenicol acetyltransferase genes, respectively, for selection with ampicillin and chloramphenicol in E. coli. The pADGAL4-2.1 and pBD-GAL4 Cam phagemid vectors contain the 2μ origin for replication. For selection in yeast, the pAD-GAL4-2.1 phagemid vector contains the LEU2 gene and the pBD-GAL4 Cam phagemid vector contains the TRP1 gene. The hybrid protein is expressed by the alcohol dehydrogenase 1 (ADH1) promoter (P ADH1) and is terminated by the ADH1 terminator (T ADH1).