Segmental isotopic labeling of a single‐domain globular protein without any refolding step by an asparaginyl endopeptidase

Asparaginyl endopeptidases (AEPs) catalyze head‐to‐tail backbone cyclization of naturally occurring cyclic peptides such as cyclotides, and have become an important peptide‐engineering tool for macrocyclization and peptide ligation. Here, we report efficient protein ligation in trans by mimicking efficient backbone cyclization by an AEP without any excess of reactants. We demonstrate a practical application of segmental isotopic labeling for NMR studies of a single‐domain globular protein without any refolding step using the recombinant AEP prepared from Escherichia coli. This simple protein ligation approach using an AEP could be applied for incorporation of various biophysical probes into proteins as well as post‐translational production of full‐length proteins.