Colicin N- an insight into a new target for antibiotics

e coli

Difficulty- 3

The discovery of Penicillin by Alexander Fleming in 1928 is hailed by many as a turning point in antibiotic technology.  Since then a number of different antimicrobial drugs have been made. However, due to bacteria growing resistant to these medications we are fast running out of ways to kill them.  MRSA and C. Difficile are two of a growing number of organisms that you may have heard of which are resistant to antibiotics, and as time goes on this list can only get longer.  Many of these drugs target lipids in the cell membranes of bacteria, so finding new targets of action that don’t attack the membrane is a big deal for scientists!  Excitingly, researchers at Newcastle University may have done just that.  A recent discovery published in June this year showed that the N-terminal domain of Colicin N, previously thought to have no cytotoxic activity, has a unique method of killing bacteria not found in other Colicins (or current antibiotics!).  This is a simplistic diagram of the different sections of Colicin N:

 colicin n

Colicins are a group of toxins which E.coli release into their environment to kill other E.coli (this is useful when competing for the same food source).  It was previously thought that the C-terminal domain (P) was the cytotoxic “killing” domain of the protein; whilst this is true, the N-terminal also contains its own unique killing mechanism.  Some substances released from organisms to kill bacteria (also known as antimicrobial peptides or AMPs) do so by folding at the membrane and assembling to form pores. Unfortunately, human membranes contain phospholipids which are also found in bacteria and so in targeting these you may also damage the host cells’ membranes.  This is why it’s so exciting that Colicin N-T can kill bacteria without binding to the membrane lipids, and that they instead target two membrane proteins: OmpF and TolA.  OmpF (an outer membrane protein) and TolA (found in the periplasm) are present in many Gram negative bacteria (see diagram below), so antibiotics that could harness this specificity could give us a new way to fight the war on germs!

Some of the more sciency points made by the research paper:

  • The C terminal domain of Colicin N (Colicin N-TRP) is not required to kill bacteria
  • Colicin N binds OmpF via a phenylalanine residue found in a region of its N terminal termed OBS1.  This can be deleted in full length Colicin N but not in Colicin N-TR/ Colicin N-T
  • AMPs often fold at inner membranes and disrupt them by assembling to form pores.  ColN-T does not form a structured peptide when near to the membrane, demonstrating another mechanism of action not involving the lipid bilayer.  This could be used as a target for new antibiotics
  • Both the T and R domains compete for OmpF binding, suggesting that there are two OmpF binding sites in ColN

I’ll finish this blog off with a quote from Professor J.H.Lakey, one of the researchers involved in the paper, who was kind enough to comment on this new and exciting discovery:

“Mostly science is about making little steps towards your goal but, now and again, you do what should be a simple everyday experiment and the results are entirely unexpected. So it was here, bacteria being killed by a “ harmless” peptide, that’s a huge leap forward and, while we are back again to the little steps, we  know  that we are a lot closer to making a useful antibiotic.”

Thanks for reading!

Johnson CL, Ridley H, Pengelly RJ, Salleh MZ, & Lakey JH (2013). The unstructured domain of colicin N kills Escherichia coli. Molecular microbiology, 89 (1), 84-95 PMID: 23672584

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