Magassa, A. | bioRxiv Science

bioRxiv2024

A platform for mapping reactive cysteines within the immunopeptidome

The major histocompatibility complex class I (MHC-I) antigen presentation pathways play pivotal roles in orchestrating immune responses. Recent studies have begun to utilize cysteines within the immunopeptidome for therapeutic applications, such as using covalent ligands to create haptenated neoantigens for inducing an immune response. In this study, we report a platform for mapping reactive cysteines present on MHC-I-bound peptide antigens. We have developed cell-impermeable sulfonated maleimide probes capable of effectively capturing reactive cysteines on antigens. Utilizing these probes in chemoproteomic experiments, we discovered that cysteines on MHC-I-bound antigens exhibit various degrees of reactivity. Furthermore, interferon-gamma stimulation produces increased reactivity of cysteines at position 8 of 9-mer MHC-I-bound antigens. Our findings may open up new avenues for understanding the distinctive roles of cysteine within the MHC-I immunopeptidome and leveraging the differentially reactive cysteines for therapeutic intervention.

systems biology↗

bioRxiv2023

A CRISPR activation screen identifies FBXO22 as an E3 ligase supporting targeted protein degradation

Targeted protein degradation (TPD) represents a potent chemical biology paradigm that leverages the cellular degradation machinery to pharmacologically eliminate specific proteins of interest. Although multiple E3 ligases have been discovered to facilitate TPD, there exists a compelling requirement to diversify the pool of E3 ligases available for such applications. This expansion will broaden the scope of potential protein targets, accommodating those with varying subcellular localizations and expression patterns. In this study, we describe a CRISPR-based transcriptional activation screen focused on human E3 ligases, with the goal of identifying E3 ligases that can facilitate heterobifunctional compound-mediated target degradation. This approach allows us to address the limitations associated with investigating candidate degrader molecules in specific cell lines that either lack or have low levels of the desired E3 ligases. Through this approach, we identified a candidate proteolysis-targeting chimera (PROTAC), 22-SLF, that induces the degradation of FKBP12 when the FBXO22 gene transcription is activated. 22-SLF induced the degradation of endogenous FKBP12 in a FBXO22-dependent manner across multiple cancer cell lines. Subsequent mechanistic investigations revealed that 22-SLF interacts with C227 and/or C228 in FBXO22 to achieve the target degradation. Finally, we demonstrated the versatility of FBXO22-based PROTACs by effectively degrading another endogenous protein BRD4. This study uncovers FBXO22 as an E3 ligase capable of supporting ligand-induced protein degradation through electrophilic PROTACs. The platform we have developed can readily be applied to elucidate protein degradation pathways by identifying E3 ligases that facilitate either small molecule-induced or endogenous protein degradation.

biochemistry↗