Molecular imaging enabled by bioorthogonal click labeling.

Abstract

Efficient protein labeling with minimal linkage error is a key requirement for super-resolution fluorescence microscopy. Many commonly used labeling strategies, including antibodies, fluorescent proteins, and self-labeling enzymes are limited by steric hindrance, and therefore they limit labeling density or constrain labeling to protein termini, thereby restricting achievable resolution at the molecular scale. Here, we describe a practical and broadly applicable protocol for site-specific protein labeling based on genetic code expansion and bioorthogonal inverse electron-demand Diels-Alder click chemistry. The method relies on the incorporation of a strained alkene-modified noncanonical amino acid at a defined position within a protein of interest, followed by rapid and selective covalent labeling with tetrazine-conjugated organic fluorophores. This approach enables the attachment of small, bright dyes with minimal linkage error and is compatible with both live-cell and fixed-cell imaging. The protocol provides detailed guidance on the design of suitable click sites, expression of amber mutants in mammalian cells, selection of appropriate tetrazine dyes, and optimization of labeling conditions for super-resolution microscopy, including single-molecule localization microscopy. Critical parameters, common pitfalls, and limitations are discussed to facilitate robust implementation across different protein classes and experimental systems. This workflow supports high-density, stoichiometric labeling and enables molecular-scale imaging of proteins in their native cellular context.