Protease is abundant in nature and has a wide variety,playing a vital role in a series of cell processes.Most proteases cause protein degra-dation by cleaving substrate sequence within the target or altering proteins into a biologically active mature form.In addition,some proteases also have special functions.For instance,the ligase-type asparagine endopeptidases (AEPs) have been recently implicated in catalyzing precursor peptides to form cyclotides,such as the well-studied prototypical cyclotide,kalata B1 (kB1) from Oldenlandia affinis encoded by the Oak1 gene [1].Backbone cyclization renders the cyclotides extreme stability,thus enabling them to be applied as ideal drug design frameworks in pharmaceutical engineering [2].Therein,it would be of great benefits if such AEPs could be devel-oped as biotechnological tools to catalyze the cyclization of peptides or proteins.