Visible Immunoprecipitation (VIP) Assay: a Simple and Versatile Method forVisual Detection of Protein-protein Interactions.

The visible immunoprecipitation (VIP) assay is a convenient alternative to conventional co-immunoprecipitation ( Katoh et al., 2015 ). By processing lysates from cells co-expressing GFP-fusion and RFP-fusion proteins for immunoprecipitation with GST-tagged anti-GFP Nanobody and glutathione-Sepharose beads, protein-protein interactions can be visualized by directly observing the beads bearing immunoprecipitates under a fluorescence microscope. This assay can examine a large number of protein combinations at one time, without requiring time-consuming procedures, including SDS-PAGE and immunoblotting. Furthermore, the VIP assay can examine complicated one-to-many and many-to-many protein interactions. Another important point of the VIP assay is the use of nanobodies for immunoprecipitation. A Nanobody is a single-domain antibody derived from Camelidae (camels and relatives). Because of its small size, high-affinity, high-specificity, and stability, anti-GFP Nanobody expressed in E. coli can be purified on a large scale, and used virtually inexhaustibly for immunoprecipitation experiments. Here we describe protocols for preparation of GST-tagged anti-GFP Nanobody and the VIP assay.