After 72 hours, less than 1% of the HEK 293H cells expressed GFP (Supplement, Fig. yeast two-hybrid system into mammalian cells [4], so do most (if not all) of commercial mammalian two-hybrid systems. One of the issues using the GAL4-based mammalian two-hybrid system is the high-level background from the genetic assays, especially when the dissociation constant (Kd) of the interacting proteins under study is above 1 M. A different protein-fragment complementation assay (PCA) has also been recently developed and used to detect protein-protein interactions in mammalian cells [5]. Since this assay is based on the stoichiometric association of two expressed fragments of a reporter protein to restore its function, the sensitivity of this assay is potentially limited by the expression level of the reporters in mammalian cells. Herein, we report a mammalian two-hybrid system (termed as trM2H) with high sensitivity and low-level background. The system can be used to detect protein-protein and peptide-peptide interactions in mammalian cells with dissociation constants (Kd) ranging from 0.99 nM to 55 M. The trM2H system can also be disrupted by the small molecule doxycycline (Dox). The design of the trM2H system is based upon the reconstruction of the functional tetracycline repressor (TetR). TetR is a transcriptional regulator and its dimeric form binds specifically to the tet operator (tetO) to inhibit the downstream gene expression of tetracycline resistance genes [6]. The dimerization of TetR is controlled by the specific interaction of its C-terminal dimerization domain (DD) [7]. In a commercial Tet-OffAdvanced Inducible Gene Expression System (Clontech), TetR is connected to three nominal transcriptional activation domains (AD) taken from herpes simplex virus VP16 [8,9]. Dimerization of the TetR-VP16 fusion allows the N-terminal TetR to bind to the Tet-Responsive Element (TREmod) while the C-terminal VP16 is free to recruit the transcriptional machinery [10,11]. This system was designed to tightly regulate the expression of protein levels in mammalian cells with the ratio of signal to noise reaching 1000:1 in MK-8998 the absence of tetracycline [8,12]. Our approach is to replace the TetR C-terminal dimerization domain (8-10) with bait and prey molecules respectively. When the bait and prey interact, the resulting TetR-VP16 will dimerize to reconstitute the functional transcriptional MK-8998 activator and initiate transcription followed by over-expression of a downstream reporter gene (GFP in this case). In contrast, absence of GFP signal indicates that there is no interaction between bait and prey molecules and the monomeric TetR-VP16 fails to initiate the transcription of GFP gene (Figure MK-8998 1). == Figure 1. == General scheme for the mammalian two-hybrid system (trM2H). Replacement of the dimerization domain (DD) of Tetracycline repressor (TetR) with interacting bait and prey molecules will yield a functional transcriptional regulator. A small molecule doxycycline can disrupt the TetR dimer and turn off the function of transcription activator and the expression of the downstream reporter protein. To locate the minimal region within TetR that is responsible for dimerization, the C-terminal -helices (1 to 10) of TetR-VP16 were sequentially deleted and replaced with homodimeric GCN4 leucine zipper (GCN4LZ), which has a Kdof 0.5 M [13,14]. The effectiveness of trM2H system was studied using fluorescence microscopy, based on the expression of GFPin vivo. In a typical experiment, human embryonic kidney (HEK) 293H cells were transfected with pTRE-Tight-AcGFP (Clontech) and pTetOff mutants at 7080% confluency using FuGene6.Figure 2A and 2Hshow that native TetR dimerizes and initiates the expression of GFP. By replacing 8-10 with GCN4LZ dimerization of TetR-VP16, the expression of GFP was restored (Fig. 2B, left lane of 2I). To address the possibility of non-specific binding (background), a GCN4LZ mutant that cannot dimerize replaced 8-10 helices of TetR [15]. As expected, this mutant failed to dimerize TetR-VP16, and no GFP was expressed (Fig. 2C, right lane Rabbit polyclonal to ADCK2 of 2I). Deletion of other helices either completely destroyed the functions of TetR-VP16 or were unable to dissociate the TetR dimer (data not shown). This set of experiments not only identified the multiple cloning site (MCS) that permits incorporation of prey and bait molecules to the C-terminus of TetR-VP16, but also demonstrated that interaction between prey and bait can reconstruct the functional TetR-VP16 transcription activator. == Figure 2. == Expression of GFP signals in a mammalian two-hybrid program was supervised by fluorescence microscopy (A-G) and Traditional western blot (H-K, probed with anti-GFP antibody)..