A detailed study has shown how a natural antibiotic changes from an inactive to active form that has an unusual lasso structure. 

Researchers David Clarke and Dominic Campopiano at the University of Edinburgh, UK, have looked in detail at the maturation process to form the active peptide antibiotic, microcin J25. The team have shown that the enzymes used to modify the peptide are associated with the bacterial inner membrane and that during the process a highly-stable lasso structure is formed. Campopiano talks more about his work in the short interview below.

1. Could you explain the significance of your article to the non-specialist? 

Microcin J25 (MccJ25) is a natural peptide antibiotic produced by specific strains of the bacterium E. coli. The peptide acts by blocking RNA-polymerase - the essential enzyme which is responsible for converting DNA information into RNA, which is then translated into protein. The MccJ25 peptide is made in an inactive form and activated after a series of chemical and structural modifications, which mature the longer precursor peptide, McjA, into microcin MccJ25. This maturation process includes an unusual protein cyclisation which produces 'a threaded lasso' structure. The chemical mechanism of this extraordinary reaction is unknown. In this work we show, for the first time, the in vitro  maturation of the McjA precursor peptide into active microcin MccJ25 by an extract containing two converting proteins, McjB and McjC. 

2. What motivated you to undertake this work? 

The overriding theme of our research is studying the biosynthesis of interesting and unusual natural products. The biosynthesis of this unique peptide 'knot' has intrigued us since its structure was elucidated in 2003 by three different research groups. 

3. Where do you see this work developing in the future? What challenges might there be? 

We have been involved with this research now for just over two years. The ultimate aim of this study is to understand the molecular details of how nature 'ties a knot' in a protein. This will involve a detailed biochemical and structural study of the two enzymes involved in microcin J25 biosynthesis, McjB and McjC. They appear to be membrane-associated and these types of protein are particularly challenging to isolate and study by structural methods (NMR, x-ray crystallography). We also aim to study the 3D structure of the McjA precursor.