Scientists figured out a way to make the human antimicrobial peptide LL‑37 in bacteria, label it for detailed studies, and pull it out using a simple formic‑acid cut. The lab‑made peptide works just as well as the chemically‑synthesized version, and the technique can be used for other small peptides that have an Asp‑Pro link.

Antimicrobial peptide LL-37 plays an important role in human body's first line of defense against infection. To better understand the mechanism of action, it is critical to elucidate the three-dimensional structure of LL-37 in complex with bacterial membranes. We present a bacterial expression system that allows the incorporation of (15)N and other isotopes into the polypeptide for nuclear magnetic resonance (NMR) analysis. The DNA sequence encoding full-length LL-37 was chemically synthesized and cloned into the pET-32a(+) vector for protein expression in Escherichia coli strain BL21(DE3). The peptide was expressed directly as a His-tagged fusion protein without the inclusion of its precursor sequence. LL-37 was released from the fusion by formic acid cleavage at the AspPro dipeptide bond and separated from the carrier thioredoxin by affinity chromatography and reverse-phase HPLC. The peptide was identified by polyacrylamide gel electrophoresis and further confirmed by mass spectrometry and NMR spectroscopy. Antibacterial activity assays showed that the recombinant LL-37 purified from the bacterial source is as active as that from chemical synthesis. According to the antimicrobial peptide database (), 111 peptides contain a Met residue, but only 5 contain the AspPro pair, indicating a broader application of formic acid than cyanogen bromide in cleaving fusion proteins. The successful application to the expression of the 66-residue cytoplasmic tail of human MUC1 indicates that the system can be applied to other peptides as well.