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Homogenous protein conjugates are required for many biological and therapeutic applications. A collection of articles in this Focus highlights some of the latest advances in developing new site-selective reactions for modifying proteins.
The modification of proteins with fluorophores, drugs and polymers is required for many applications, yet conjugation reactions often generate a heterogeneous mixture of products. A collection of articles in this issue focuses on methods to modify proteins in a site-selective manner.
The discovery of a tetrapeptide containing a reactive cysteine provides a method to site-selectively modify peptides and proteins, even if other cysteine residues are present in the polypeptide chain.
A wide range of different aqueous chemistries for the site-selective modification of proteins have been described over the past decade. This Perspective discusses the scope and potential of chemical site-selective protein-modification methods in the context of their biological and therapeutic applications.
Antibody–drug conjugates have shown considerable promise for treating disease. However, in order to deliver their full potential, sophisticated site-specific conjugation technologies are needed. This Perspective provides an overview of the different methods used for the site-specific attachment of cytotoxic agents to antibodies.
Incorporation of a π-clamp—a four-residue sequence (Phe-Cys-Pro-Phe)—into a protein enables the site-specific modification of the π-clamp cysteine side-chain. The π-clamp can be genetically encoded and does not require protecting-groups or catalysts to provide selective conjugation.
The rapid and selective regulation of a target protein within living cells containing closely related family members is a longstanding challenge. Now the introduction of genetically directed bioorthogonal ligand tethering (BOLT) and the demonstration of selective inhibition (iBOLT) and optical switching (photo-BOLT) of protein function in live mammalian cells addresses this challenge.
A method for engineering site-specific modifications of histone proteins within cellular chromatin has been developed using protein trans-splicing. This approach enabled a native histone modification, H2BK120 ubiquitination, to be incorporated in isolated nuclei, which was shown to trigger a downstream epigenetic effect.
Conjugation of DNA to proteins often involves a choice between either expressing recombinant proteins with a specific handle, or labelling wild-type proteins with low site-selectivity. Now preorganization of a DNA–ligand complex to a metal-binding site enables site-selective conjugation of a DNA strand to lysine residues of wild-type proteins and antibodies.
A series of quadruplet decoding tRNAs has been developed to form an optimized orthogonal translation system. These tRNAs enable efficient, site-specific incorporation of multiple unnatural amino acids into a protein, with a substantial increase in yield over previous methods. The amino acids are then used to site-specifically label a protein with a pair of fluorophores, enabling studies of the protein's dynamics.
Restoring a protein's function in response to specific stimuli can enable a signalling pathway to be activated and the effect monitored over time. Here, a chemical rescue strategy for restoring protein function inside live cells is described, in which palladium catalysts are used to deprotect a propargylcarbamate group of a lysine analogue.
The site-specific incorporation of a norbornene amino acid into proteins via genetic code expansion, together with the synthesis of a series of tetrazine-based probes that exhibit turn-on fluorescence on their fast cycloaddition with norbornene, enables rapid protein labelling on mammalian cells.