[Development of specific protein labeling method by using small molecular probes].

Selective protein labeling with a small molecular probe is a versatile method for elucidating protein functions in living cells. In this review, we report a new protein labeling method that enables selective covalent modification of a tag-fused protein with small functional molecules. This method utilizes the specific interaction and rapid reaction between a short peptide tag and a molecular probe, which comprises the cysteine-containing short CA6D4x2 tag (CAAAAAADDDDGDDDD) and a tetranuclear Zn(II)-DpaTyr probe containing a reactive α-chloroacetyl moiety. This labeling system, so-called reactive tag system, was successfully applied to the fluorescence imaging of tag-fused GPCRs such as bradykinin receptor (B2R) and acetylcholine receptor (m1AchR) expressing on HEK293 cells. The utility of this labeling method was demonstrated in the function analyses of GPCRs, such as fluorescence visualization of the stimuli-responsive internalization of GPCRs and pH change in endosomes containing the internalized GPCRs.