The formation of pathological protein aggregates occurs in many neurodegenerative diseases, such as the aggregation of the protein tau in patients with Alzheimer’s disease. Removing these aggregates offers a possible route to treat these protein misfolding diseases; however, targeting the aggregated form of a protein while sparing the monomeric form, which is required for normal cellular function, remains a challenge. Now, Benn, Cheng et al. have shown that fusing an E3 ligase-containing protein to a target-specific nanobody enables the construction of RING–nanobody (R-Nb) degraders that can target and selectively remove proteins that have assembled into protein aggregates.

The E3 ubiquitin ligase tripartite motif-containing protein 21 (TRIM21) induces degradation by the clustering and cross-activation of the RING domains of TRIM21. The team hypothesized that the structure of fibrillar aggregates may enable dense clustering of R-Nb degraders and therefore activation of the degradation pathway, whereas monomeric tau would not induce clustering of R-Nbs and therefore would not induce degradation. Tuning the affinity of the nanobody could also affect clustering and thereby provides another route to mediate protein degradation.