Expressed Protein Ligation without Intein.

Proteins with a functionalized C-terminus such as a C-terminal thioester are key to the synthesis of larger proteins via expressed protein ligation. They are usually made by recombinant fusion to intein. Although powerful, the intein fusion approach suffers from premature hydrolysis and low compatibility with denatured conditions. To totally bypass the involvement of an enzyme for expressed protein ligation, here we showed that a cysteine in a recombinant protein was chemically activated by a small molecule cyanylating reagent at its N-side amide for undergoing nucleophilic acyl substitution with amines including a number of l- and d-amino acids and hydrazine. The afforded protein hydrazides could be used further for expressed protein ligation. We demonstrated the versatility of this activated cysteine-directed protein ligation (ACPL) approach with the successful synthesis of ubiquitin conjugates, ubiquitin-like protein conjugates, histone H2A with a C-terminal posttranslational modification, RNase H that actively hydrolyzed RNA, and exenatide that is a commercial therapeutic peptide. The technique, which is exceedingly simple but highly useful, expands to a great extent the synthetic capacity of protein chemistry and will therefore make a large avenue of new research possible.

E1-catalyzed ubiquitin C-terminal amidation for the facile synthesis of deubiquitinase substrates.

Will Ub my partner? The ubiquitin (Ub)-activating enzyme (E1) was used to catalyze an amidation reaction to functionalize the C terminus of Ub with unique functional groups, such as thiol, azide, alkyne, and alkene groups, with high efficiency and yield (>90 %). These groups were then applied for the facile synthesis of fluorophore-conjugated ubiquitin and specifically conjugated diubiquitin substrates for deubiquitinases.