Semisynthetic Viral Inhibitor for Light Control of the MHC I Peptide Loading Complex

The immune system detects virally or malignantly transformed cells via peptide‐loaded major histocompatibility complex class I (pMHC I) molecules on the cell surface. MHC I molecules are loaded with cargo peptides in the endoplasmic reticulum (ER) by the highly dynamic multiprotein peptide loading complex (PLC). Here, we developed a semisynthetic approach to generate a photocleavable immune modulator ICP47 of Herpes simplex virus. Using this nanotool, we revealed key mechanistic events of the purified PLC, such as peptide binding and translocation coupled to ATP hydrolysis, triggered by light. We established a single‐organelle flow cytometry assay to monitor light‐controlled activation of the antigen processing machinery in native ER membranes. This photochemical modulation opens new opportunities for a comprehensive mechanistic analysis of the antigen processing machinery in vitro and native membrane environment.