Chloe Mitchell is a Ph.D. Candidate in Charles Debers lab at the University of Toronto. She is studying new mechanisms to tackle multidrug resistant bacteria

Targeting membrane-embedded protein-protein interactions continues to be a challenge. We have employed a rational drug design to target the bacterial Small Multidrug Resistant, SMR, efflux protein, which homodimerizes through its 4^th transmembrane helix, TM4. The peptide inhibitors contain the TM4 Gly-Gly heptad dimerization motif, to align with –and competitively disrupt– the membrane-embedded TM4-TM4 interface, and thereby disable substrate efflux. The peptide inhibitors also contain two tags: a C-terminal positively-charged Lys tag to direct the peptide to the negatively-charged bacterial membrane, and a non-charged N-terminal peptoid tag to promote membrane insertion.

The designed peptides have the prototypical sequence Ac-A-(Sar)₃-XXGIXLIXXGVXX-KKK, Sar = N-methyl-Gly. We have earlier shown that these peptides reduce SMR mediated efflux activity, ostensibly through the specifically designed mechanism. However, the architecture of these peptides resembles that of cationic antimicrobial peptides, as both categories of peptides have a hydrophobic and charged domain. Therefore, we explored in structural detail the effects inhibitor peptides have on the bacterial membrane.

Through circular dichroism and fluorescence spectroscopy, we first confirmed that these peptides are inserted and helical in a membrane environment. To determine effects of insertion, we used in vivo and in vitro dye-based assays and found that the peptides displayed minimal disruption of the membrane per se.

Interestingly, peptide insertion in absence of protein was seen to cause apparent membrane reorganization in liposomes, 3:1 POPE:POPG, as assessed through dual peaks detected by differential scanning calorimetry. Our results show promise for a new approach to target and disrupt membrane protein-protein interactions.