The long-term stability of affinity-based protein labeling probes is crucial to obtain reproducible protein labeling results. However, highly stable probes generally suffer from low protein labeling efficiency and pose significant challenges when labeling low abundance native proteins in living cells. In this paper, we report that protein labeling probes based on an ortho-difluorophenyl ester reactive module exhibit long-term stability in DMSO stock solution and aqueous buffer, yet they can undergo rapid and selective labeling of native proteins. This novel electrophile can be customized with a wide range of different protein ligands and is particularly well-suited for the labeling and imaging of transmembrane proteins. With this probe design, the identity and relative levels of basal and hypoxia-induced transmembrane carbonic anhydrases were revealed by live cell imaging and in-gel fluorescence analysis. We believe that the extension of this difluorophenyl ester reactive module would allow for the specific labeling of various endogenous membrane proteins, facilitating in-depth studies of their distribution and functions in biological processes.

To date, various affinity-based protein labeling probes have been developed and applied in biological research to modify endogenous proteins in cell lysates and on the cell surface. However, the reactive groups on the labeling probes are also the cause of probe instability and nonselective labeling in a more complex environment, e. g., intracellular and in vivo. Here, we show that labeling probes composed of a sterically stabilized difluorophenyl pivalate can achieve efficient and selective labeling of endogenous proteins on the cell surface, inside living cells and in vivo. As compared with the existing protein labeling probes, probes with the difluorophenyl pivalate exhibit several advantages, including long-term stability in stock solutions, resistance to enzymatic hydrolysis and can be customized easily with diverse fluorophores and protein ligands. With this probe design, endogenous hypoxia biomarker in living cells and nude mice were successfully labeled and validated by in vivo, ex vivo, and immunohistochemistry imaging.

GPI-anchored folate receptor α (FRα) is an attractive anticancer drug target and diagnosis marker in fundamental biology and medical research due to its significant expression on many cancer cells. Currently, analyses of FRα expression levels are usually achieved using immunological methods. Due to the continual FRα synthesis and degradation, immunological methods are not suitable for studying real-time dynamic activities of FRα in living cells. In this paper, we introduce a rapid and specific FRα protein-labeling fluorescent probe, FR1, to facilitate the study of the dynamics of expression and degradation processes of endogenous FRα in living cells. With this labeling probe, insights on FRα protein lifetime and shedding from the cell surface can be obtained using fluorescence live-cell imaging and electrophoresis techniques. We revealed that FRα undergoes soluble domain release and endocytosis degradation simultaneously. Imaging results showed that most of the membrane FRα are transported to the lysosomes after 2 h of incubation. Furthermore, we also showed that the secretion of a FRα soluble domain into the environment is most likely accomplished by phospholipase. We believe that this protein-labeling approach can be an important tool for analyzing various dynamic processes involving FRα.

Currently most fluorogenic probes are developed for the analysis of enzymes, where a bond breaking or rearrangement reaction is required to transform a nonfluorescent enzymatic substrate into a fluorescent product. However, this approach cannot be used for proteins that do not possess enzymatic activities. In this article, we show that fluorogenic probes with a self-immolative difluorophenyl ester linker can mimic the bond disassembly processes of fluorogenic enzyme substrates for the rapid analysis of nonenzymatic proteins. Although numerous self-immolative reagents have shown promising applications in sensors, drug delivery systems, and material chemistry, all of them are triggered by either enzymes or small reactive molecules. In our strategy, the probe binds to the protein via a specific protein–ligand interaction, inducing a chemical reaction between the self-immolative linker and an amino acid of the protein, thereby triggering a cascade reaction that leads to the activation and release of the fluorogenic reporter. In contrast, a phenyl ester linker without the difluoro substituent cannot be triggered to release the fluorogenic reporter. With this probe design, live-cell imaging of extracellular and intracellular endogenous tumor marker proteins can be achieved with high selectivity and sensitivity.