Reducing the level of microtubule-associated protein tau has recently emerged as a promising therapeutic approach for a range of neurodegenerative diseases. Among the various approaches, targeted protein degradation provides a reversible means to rapidly reduce and specifically target disease-relevant forms of tau. However, in aging cells, the protein turnover activity is generally weakened, reducing the efficacy of protein degradation. A potential solution to this is to harness the nuclear

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... easomal activity. The nucleus has a high proteasomal content and the degradation activity remains relatively unaffected even in aged cells. Here we show that an E3 ligase F-box domain from the nuclear protein human speckle type BTB/POZ protein (SPOP) is effective in degrading the microtubule-associated protein tau in primary mouse hippocampal neurons. Using EGFP-tagged tau and a GFP-binding nanobody fused to SPOP, we found that the native nuclear localization signal in SPOP causes nuclear sequestration of the target protein. However, degradation of the sequestered target proteins is incomplete, resulting in nuclear accumulation. Replacing the native SPOP nuclear localization signal (NLS) with variants having altered nuclear localization efficiency dramatically affects in the degree of nuclear accumulation of the target protein. Interestingly, nanobody-SPOP with no NLS was more efficient than that with a NLS in reducing overall tau level, causing an approximately 50% reduction in ectopically expressed human tau in mouse neurons. These results show the potential for harnessing the nuclear proteasomal activity for targeted tau degradation in cells and demonstrate a new modality of regulating intracellular protein degradation.