Chem. and biol. labeling is fundamental for the elucidation of the function of proteins within biochem. cellular networks. In particular, fluorescent probes allow detection of mol. interactions, mobility and conformational changes of proteins in live cells with high temporal and spatial resoln. We present a generic method to label proteins in vivo selectively, rapidly (seconds) and reversibly, with small mol. probes that can have a wide variety of properties. These probes comprise a chromophore and a metal-ion-chelating nitrilotriacetate (NTA) moiety, which binds reversibly and specifically to engineered oligohistidine sequences in proteins of interest. We demonstrate the feasibility of the approach by binding NTA-chromophore conjugates to a representative ligand-gated ion channel and G protein-coupled receptor, each contg. a polyhistidine sequence. We investigated the ionotropic 5HT3 serotonin receptor by fluorescence measurements to characterize in vivo the probe-receptor interactions, yielding information on structure and plasma membrane distribution of the receptor. [on SciFinder (R)]