4.2.8. Mass Spectrometry

Maldi TOF MS analysis was performed on a Bruker Ultraflextreme MALDI-TOF/TOF instrument (Bruker Daltonics, Billerica, MA, USA) by the Centro Piattaforme Tecnologiche of the University of Verona as previously described [30].

**Author Contributions:** Conceptualization, M.D., F.M. and M.A.; investigation, F.M., C.G.B., F.P. and R.T.; formal analysis, L.B.; writing—original draft preparation, F.M.; writing—review and editing, M.D. and M.A.; supervision, M.D. and M.A.; funding acquisition, M.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Alzheimer's Association, gran<sup>t</sup> number AARG-17-529221 awarded to MD, and by the University of Verona, Progetto Ricerca di Base 2015 awarded to MD.

**Acknowledgments:** FM thanks "Fondazione Umberto Veronesi" for granting a postdoctoral fellowship. We thank Centro Piattaforme Tecnologiche of the University of Verona for providing access to the Mass Spectrometry Platform, and David Fushman who kindly provided the plasmids for expression of recombinant E2-25K, Ubc13 and Mms2.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
