Real-time monitoring of cell surface protein arrival with split luciferases

Each cell in a multicellular organism permanently adjusts the concentration of its cell surface proteins. In particular, epithelial cells tightly control the number of carriers, transporters and cell adhesion proteins at their plasma membrane. However, the sensitively measuring the cell surface concentration of a particular protein of interest in live cells and in real time represents a considerable challenge. Here, we introduce a novel approach based on split luciferases, which uses one luciferase fragment as tag on the protein of interest and the second fragment as supplement to the extracellular medium. Once the protein of interest arrives at the cell surface, the luciferase fragments complement and generate luminescence. We compared the performance of split Gaussia luciferase and split Nanoluciferase by using a system to synchronize biosynthetic trafficking with conditional aggregation domains. The best results were achieved with split Nanoluciferase, for which luminescence increased more than 6000-fold upon recombination. Furthermore, we showed that our approach can separately detect and quantify the arrival of membrane proteins at the apical and basolateral plasma membrane in single polarized epithelial cells by detecting the luminescence signals with a microscope, thus opening novel avenues for characterizing the variations in trafficking in individual epithelial cells. Synopsis We present here a novel method to determine the arrival of a membrane protein of interest at the cell surface in real time and with sufficient sensitivity to achieve single-cell resolution. This allows measuring detailed kinetics of plasma membrane trafficking and thus to detect also minor changes in protein sorting and intracellular trafficking. Furthermore, the single-cell sensitivity of the method will enable to systematically characterize variations in trafficking between neighboring cells within a multicellular organism. ### Competing Interest Statement The authors have declared no competing interest.