**Acknowledgements**

nanobody-based methods rely on the interaction between a GFP Nb and a GFP-tagged protein. Herce et al. covalently linked a GFP Nb with a protein that accumulates at a specific subcellular location. In mammalian cells, this protein could be, for example, laminin B1 or centrin, which results in the delocalization of the GFP Nb to the nuclear lamina or the cytoplasmic centrioles, respectively. Subsequently, a GFP-tagged protein will be recruited to a specific location. If the second protein of interest, labeled with another fluorophore, interacts with the first protein of interest, the fluorophores will co-localize at a discrete spot. This interaction can be visualized by a single-fluorescence snapshot. Interestingly, this technique also allows screening for inhibitors of protein-protein interactions [84]. Another recently developed technique uses biocompatible engineered upconversion nanoparticles (UCNPs) conjugated with GFP Nbs. Visualization of the interaction between two proteins of interest is based on the lanthanide resonance energy transfer (LRET). As a proof of concept, they probed for the indirect interaction between the mitochondrial proteins TOM20 and TOM7. The latter was expressed as a fusion protein with EGFP and the former as a fusion protein with dsRed and a Halo tag. This Halo tag was subsequently labeled with tetramethylrhodamine (TMR), while the EGFP was recognized by the GFP Nb-labeled UCNPs. Co-localization of both proteins results in the detection of LRET-sensitized TMR emission. Remarkably, TOM7 and TOM20 are spatially separated by TOM40. The capacity of this technique for reporting indirect long-

distance interactions might be of interest to unravel cellular protein complexes [85].

Nanobodies are highly versatile tools with interesting biochemical properties, which result in their application in various fields ranging from basic research and diagnostics to therapy. In this chapter, we aim to shed light on their multifunctionality and in this way encourage other researchers to include this technology in their future projects. Since their discovery in 1993, the numbers of publications wherein nanobodies are employed are gradually increasing which indicate that their merit has been proved. Here, we have shown that nanobodies have a high therapeutic potential and form an ideal stepping stone to drug development. Despite isolated cases, nanobodies are not capable of traversing the cellular membrane, preventing their direct use as a therapeutic. The effects observed with nanobody treatment are established through multiple mechanisms. Nanobodies can act as an inhibitor of enzymatic activity, interfere with specific protein-protein interactions, and shield a protein of interest from aberrant cleavage, or they can be used as a tool to target proteins for proteasomal degradation. We believe that effects triggered by nanobodies in vitro or in vivo are a faithful representation of what to expect with conventional pharmacological drugs, since both compounds directly target the resident endogenous protein. However, since current experiments are often limited to cell-based assays, animal experiments are warranted to confirm their effectiveness. Furthermore, nanobodies have a lot to offer as a research tool. They can help researchers to elucidate protein functions and thereby gain insight in biological pathways. Several strategies are possible, ranging from subcellular delocalization to the induction of protein knockouts. Last but not least, nanobodies may represent an adequate answer to problems encountered with (conventional) antibody reproducibility

**4. Concluding remarks**

220 Antibody Engineering

This work was supported by grants from the Research Foundation Flanders (Fonds Wetenschappelijk Onderzoek (FWO) Vlaanderen) and Ghent University (BOF13/GOA/010). AS and LB are supported by the Agency for Innovation by Science and Technology in Flanders (IWT-Vlaanderen). We apologize to those researchers whose work could not be cited.
