Labeling of proteins using a naturally split intein

Inteins are internal polypeptide sequences, which excise themselves out of a precursor protein. During this protein splicing process the flanking protein sequences, the so-called exteins, are linked via a native peptide bond.^[1] In comparison to cis-protein splicing with only one precursor protein, protein splicing can also occur in trans with two precursor proteins. The split inteins that catalyze trans-protein splicing can be found in nature but can also be generated artificially. In the trans-splicing process both intein halves, the Int^N and Int^C, first associate to form an active complex before the splicing reaction can occur. This enables separate synthesis and/or modification of the two exteins and thereby allows a broad range of chemical modifications useful for many applications in protein and cellular chemistry.^[2]

The naturally split α subunit of the DNA polymerase III (DnaE) intein from Nostoc punctiforme PCC73102 (Npu) was studied in vitro. The trans-splicing reaction proceeds with extraordinary high apparent first-order rate constants, especially at 37°C (k = [1.1 ± 0.2] x 10^-2 s^-1). Furthermore, the Npu DnaE split intein exhibits a broad tolerance towards different exteins and reaction conditions.^[3] The Npu DnaE split intein is thus superior to other natural or artificial split inteins, and holds great promise for the modification of proteins under physiological conditions. In this regard we could also show that the Npu DnaE intein can be used for efficient protein backbone manipulation of cell surface proteins on mammalian cells.^[4]