The in vivo monitoring and characterization of protein-protein interactions are essential to understanding cellular events in living organisms. Proteins never act alone in the cell; on the contrary, they associate with each other to form stable or transient complexes that execute certain cellular functions. Aberrant protein interactions were observed in many pathological conditions, and therefore in vivo investigation of protein-protein interactions-noninvasively with high temporal and spatial resolution-could shed new light on the mechanisms underlying these interactions and provide new insights for the development of new treatment strategies.
To adapt existing in vitro and cell-culture-based techniques to study protein-protein interactions in a living organism, the signal of the event must be detected noninvasively from inside the living subject. Optical imaging is an ideal tool for such research, as it allows monitoring intracellular processes in real time, without interferences, and with high spatial and temporal resolution.
Three major approaches were developed to monitor protein-protein interactions in vivo using optical reporters. These include the two-hybrid system, protein complementation assays based on luciferases and fluorescent proteins, and assays based on nonradiative energy transfer (FRET and BRET).
6.1 Two-Hybrid System for In Vivo Monitoring of Protein-Protein Interactions
The two-hybrid system exploits the ability of a pair of interacting proteins to bring a transcription activation domain into close proximity with a DNA-binding site, which regulates the expression of an adjacent reporter gene (Fig. 6.1).
This method was originally developed for yeast. Two proteins of interest were expressed in yeast: one fused to a DNA-binding domain, and the other to a transcriptional activation domain. The first hybrid protein can bind to DNA, but will not activate transcription if it does not have an activation domain. Another hybrid protein, if present alone, does not activate transcription because it does not bind to the upstream activation system. When both hybrid proteins are present, they form a complex, thus bringing both the DNA-binding domain and the transcription activation domain together, which in turn triggers the transcription of the reporter gene. In early publications, β-galactosidase was used as a reporter molecule generating blue colonies on plates or filters containing X-Gal. The two-hybrid system was shown to be highly sensitive; the level of induced transcriptional activity seemed to be proportional to the affinity of the investigated proteins. However, due to the nature of the signal (color), it was impossible to apply this method for real-time in vivo imaging.