Functional analysis of protein interactions using coupled bi-fluorescence complementation/GFP nanobody techniques.

Transcription factors (TFs) form homo- or hetero-dimeric DNA binding complexes along with associated co-regulators that can have transcriptional repressor or activator functions. Defining the specific composition of the complexes is therefore key to understanding their biological role. Here, we utilized bimolecular fluorescence complementation (BiFC) to visualize the formation of defined TF dimers and associated co-regulators derived from the activator protein-1 (AP-1) and myocyte enhancer factor 2 (MEF2) families. Firstly, BiFC signals were observed in cells co-expressing TFs tagged with complimentary combinations of the split fluorescent protein, demonstrating the engineered formation of defined dimer complexes. Next, we applied this approach and determined that defined AP-1 dimers localized at discrete sub-nuclear locations. Subsequently, a combination of BiFC coupled with GFP binding peptide (GBP)-nanotrap allowed observation of protein-protein interactions between a co-regulator, HDAC4, and defined BiFC-MEF2 engineered dimers. To determine transactivation properties of defined TF dimers in a cellular system, the Gal4-DNA binding domain fused to GBP was utilized to assess the transcriptional properties of the BiFC-TF dimers using a generically applicable Gal4/UAS luciferase reporter gene assay system. Here, we report efficacy of a BiFC/GBP-nanobody approach that allows engineering, visualization, and functional analysis of defined TF dimers.