<scp>E3</scp> ligase Praja1 mediates ubiquitination and degradation of microtubule‐associated protein tau

The RING-H2 type E3 ligase Praja family is composed of E3 ubiquitin-protein ligases Praja1 and Praja2, which promote the degradation of substrates through the ubiquitin-proteasome system. Both paralogs contribute to neuronal maturation and differentiation, indicating a significant role in the nervous system. Aggregation-prone proteins associated with neurodegenerative diseases, including TAR DNA-binding protein 43 (TDP-43) and α-synuclein, are degraded and/or suppressed by Praja1. Furthermore, the expression level of the microtubule-associated protein tau (MAPT) gene, which is frequently mutated in Alzheimer's disease, is regulated by Praja2. Although the Praja family has been shown to recognize various aggregation-prone proteins as substrates, it has not been determined whether tau, a key protein that aggregates in tauopathies, is also recognized by Praja proteins. In this study, we show that Praja1, but not Praja2, recognizes tau as a candidate substrate. We observed that the tau protein level in human neuroblastoma SH-SY5Y cells decreased depending on the E3 ligase activity of Praja1. Furthermore, the in vivo/in vitro ubiquitination assay showed that Praja1 ubiquitinates tau, indicating that it is a target substrate. Next, by combining ancestral sequence reconstruction and mutational analysis, we revealed that the Praja1-tau interaction began just after the duplication of the Praja family in the common ancestor of placentals. Lastly, to test whether this interaction is disrupted under pathological conditions, P301L tau was introduced, resulting in a degradation similar to that of wild-type tau. These results reveal an unidentified mechanism of tau proteostasis by Praja1 and may provide insight into the pathogenesis of neurodegenerative diseases, including tauopathy.