Semisynthetic and Enzyme-Mediated Conjugate Preparations Illuminate the Ubiquitination-Dependent Aggregation of Tau Protein

In the brain of individuals with Alzheimer's disease, the regulatory protein ubiquitin is found conjugated to different lysine residues of tau protein assembled into pathological paired helical filaments. To shed light on the hitherto unexplored ubiquitination-linked conformational transitions of tau, the availability of in vitro ubiquitin conjugation methods is of primary importance. In our work, we focused on the four-repeat domain of tau and assembled an enzymatic machinery formed by UBE1, Ubc13, and CHIP enzymes. The enzymatic reaction resulted in monoubiquitination at multiple sites, reminiscent of the ubiquitination pattern observed in vivo. We further exploited chemoselective disulfide coupling reactions to construct three tau regioisomers with site-specific monoubiquitination. Protein aggregation experiments revealed that the multiple enzyme-derived products were unable to convert into amyloid fibrils, while the semisynthetic conjugates exhibited diverse capability to form filaments. This study contributes novel insight into the effects of a key post-translational modification on aberrant protein self-assembly.     

A Quantitative Chemical Proteomics Approach for Site‐specific Stoichiometry Analysis of Ubiquitination

Stoichiometric analysis of post‐translational modifications is an emerging strategy for absolute quantification of the fractional abundance of the modification. Herein, a quantitative chemical proteomic workflow for stoichiometric analysis of ubiquitination is reported, named isotopically balanced quantification of ubiquitination (IBAQ‐Ub). The strategy utilizes a new amine‐reactive chemical tag (AcGG‐NHS) that is structurally homologous to the GG remnant of ubiquitin on modified lysine after trypsin cleavage and therefore enables the generation of structurally identical peptides from ubiquitinated and unmodified lysine residues following trypsin digestion and secondary stable isotopic labeling. The strategy is highly robust, sensitive, and accurate with a wide dynamic range using either protein standards or complex cell lysates. Thus, this work provides an efficient chemical proteomics tool for quantitative stoichiometric analysis of ubiquitination signaling pathways.     

DNP‐Supported Solid‐State NMR Spectroscopy of Proteins Inside Mammalian Cells

Elucidating at atomic level how proteins interact and are chemically modified in cells represents a leading frontier in structural biology. We have developed a tailored solid‐state NMR spectroscopic approach that allows studying protein structure inside human cells at atomic level under high‐sensitivity dynamic nuclear polarization (DNP) conditions. We demonstrate the method using ubiquitin (Ub), which is critically involved in cellular functioning. Our results pave the way for structural studies of larger proteins or protein complexes inside human cells, which have remained elusive to in‐cell solution‐state NMR spectroscopy due to molecular size limitations.     

Chemical Synthesis of Diubiquitin‐Based Photoaffinity Probes for Selectively Profiling Ubiquitin‐Binding Proteins

Biochemical studies of cellular processes involving polyubiquitin have gained increasing attention. More tools are needed to identify ubiquitin (Ub)‐binding proteins. We report diazirine‐based photoaffinity probes that can capture Ub‐binding proteins in cell lysates, and show that diazirines are preferable to aryl azides as the photo‐crosslinking group, since they decrease non‐selective capture. Photoaffinity probes containing at least two Ub units were required to effectively capture Ub‐binding proteins. Different capture selectivity was observed for probes containing diubiquitin moieties with different types of linkages, thus indicating the potential to develop linkage‐dependent probes for selectively profiling Ub‐binding proteins under various cellular conditions.