Structure and organization of the human antimicrobial peptide LL-37 in phospholipid membranes: relevance to the molecular basis for its non-cell-selective activity

The antimicrobial peptide LL-37 belongs to the cathelicidin family and is t he first amphipathic ct-helical peptide isolated from human. LL-37 is consi dered to play an important role in the first line of defence against local infection and systemic invasion of pathogens at sites of inflammation and w ounds. Understanding its mode of action may assist in the development of an timicrobial agents mimicking those of the human immune system. In vitro stu dies revealed that LL-37 is cytotoxic to both bacterial and normal eukaryot ic cells. To gain insight into the mechanism of its non-cell-selective cyto toxicity, we synthesized and structurally and functionally characterized LL -37, its N-terminal truncated form FF-33, and their fluorescent derivatives (which retained structure and activity). The results showed several differ ences, between LL-37 and other native antimicrobial peptides, that may shed light on its in vivo activities. Most interestingly, LL-37 exists in equil ibrium between monomers and oligomers in solution at very low concentration s. Also, it is significantly resistant to proteolytic degradation in soluti on, and when bound to both zwitterionic (mimicking mammalian membranes) and negatively charged membranes (mimicking bacterial membranes). The results also showed a role for the N-terminus in proteolytic resistance and haemoly tic activity, but not in antimicrobial activity. The LL-37 mode of action w ith negatively charged membranes suggests a detergent-like effect via a 'ca rpet-like' mechanism. However, the ability of LL-37 to oligomerize in zwitt erionic membranes might suggest the formation of a transmembrane pore in no rmal eukaryotic cells. To examine this possibility we used polarized attenu ated total reflectance Fourier-transform infrared spectroscopy and found th at the peptide is predominantly alpha-helical and oriented nearly parallel with the surface of zwitterionic-lipid membranes. This result does not supp ort the channel-forming hypothesis, but rather it supports the detergent-li ke effect.