A new method called MicroMapping can identify nanoscale protein–protein interactions on live cells.
Major Finding: A new method called MicroMapping can identify nanoscale protein–protein interactions on live cells.
Approach: The technique uses a photocatalyst–antibody conjugate that can be targeted to a protein of interest.
Impact: MicroMapping identified two potential PD-L1 interactors on cell surfaces, demonstrating its utility.
It is of interest to determine the spatial relationships between cell-surface proteins—for instance, the immune-checkpoint blockade target PD-L1—and other proteins in the vicinity. With the aim of developing a technique to precisely define spatial relationships among cell-surface proteins over short ranges, Geri, Oakley, Reyes-Robles, Wang, and colleagues developed an approach dubbed MicroMapping (µMapping). This technique relies on an iridium-based photocatalyst that can be activated by blue light to produce a short-lived carbene (a compound containing a divalent carbon atom) that, with a half-life of less than one nanosecond, can cross-link a nearby C–H bond (such as one in a protein) or be quenched by water. After optimizing the chemical properties of this photocatalytic protein-labeling agent, the compound was linked to an antibody to enable it to selectively bind a cell-surface protein of interest. Experiments applying the antibody–photocatalyst conjugate to live cells provided proof of concept that cell-surface labeling using this technique was feasible and demonstrated that the new method was capable of discriminating between unrelated microenvironments. Further experiments using live cells, this time targeting PD-L1 on B-cell surfaces, revealed proximity between PD-L1 and the previously unknown interactors CD30, a tumor-cell marker that is a member of the tumor necrosis factor family, and CD300A, an immune-inhibitory glycoprotein. PD-L1–targeted µMapping also enabled protein labeling in trans; that is, labeling was possible across the immunosynaptic junction formed by PD-L1–PD-1 interactions. The approach outlined in this study can thus be used to identify protein–protein interactions in live cells at a nanometer scale, enabling exploration of basic biological questions about topics such as signal transduction and interactions at cell–cell interfaces. Further, it may be possible to apply µMapping to more translational work with the aim of identifying interactors of cell-surface proteins especially important in cancer, such as PD-L1.
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