Ligand-Directed Acyl Imidazole Chemistry for Labeling of Membrane-Bound Proteins on Live Cells

Sho-hei Fujishima†, Ryosuke Yasui†, Takayuki Miki†, Akio Ojida*‡, and Itaru Hamachi*†

^† Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

^‡ Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/ja2108855

Publication Date (Web): February 21, 2012

Copyright © 2012 American Chemical Society

Chemistry-based protein labeling in living cells is undoubtedly useful for understanding natural protein functions and for biological/pharmaceutical applications. Here, we report a novel approach for endogenous membrane-bound protein labeling for both in vitro and live cell conditions. A moderately reactive alkyloxyacyl imidazole (AI) assisted by ligand-binding affinity (ligand-directed AI (LDAI)) chemistry allowed us to selectively modify natural proteins, such as dihydrofolate reductase (DHFR) and folate receptor (FR), neither of which could be efficiently labeled using the recently developed ligand-directed tosylate approach. It was clear that LDAI selectively labeled a single Lys(K32) in DHFR, proximal to the ligand-binding pocket. We also demonstrate that the fluorescein-labeled (endogenous, by LDAI) FR works as a fluorescent biosensor on the live KB cell surface, which allowed us to carry out unprecedented in situ kinetic analysis of ligand binding to FR.