Cellular & Molecular Immunology 21 Review Volume 3 Number 1 February 2006 The Molecular Mechanism of Interaction between Sushi Peptide and Pseudomonas Endotoxin Peng Li 1 , Miao Sun 1 , Thorsten Wohland 2 , Bow Ho 3, 4 and Jeak Ling Ding 1, 4 Septic shock is caused by Gram-negative bacterial infection. Lipopolysaccharide (LPS) is the bioactive molecule present on the outer membrane of the Gram-negative bacteria. It is generally thought that LPS interacts with sensors on the host cell membrane to activate the intracellular signaling pathway resulting in the overproduction of cytokines such as TNF-α. This causes inflammation and ultimately, septic shock. Lipid A is the pharmacophore of the LPS molecule. Thus, developing bio-molecules which are capable of binding LPS at high affinity, especially to the lipid A moiety is an efficient way to neutralize the LPS toxicity. Factor C, a serine protease in the horseshoe crab ameobocytes, is sensitive to trace levels of LPS. We have derived Sushi peptides from the LPS-binding domains of Factor C. Our earlier study showed that the Sushi peptides inhibit LPS-induced septic shock in mice. Here, we demonstrate that the molecular interaction between LPS and Sushi 1 peptide is supported by the hydrophobic interaction between the lipid tail of LPS and Sushi 1 peptide. Furthermore, in the presence of LPS, the peptide transitions from a random structure into an α-helical conformation and it disrupts LPS aggregates, hence, neutralizing the LPS toxicity. Cellular & Molecular Immunology. 2006;3(1):21-28. Key Words: septic shock, Sushi 1 peptide, Factor C, Pseudomonas endotoxin/LPS, LPS binding, electrostatic and hydrophobic interaction Introduction Gram-negative bacteria are the major pathogens responsible for a wide variety of infections and illnesses. During Gram- negative bacterial infection, the LPS, also known as endotoxin, causes excessive release of inflammatory cyto- kines, leading to multiple organ failure and death. The indomitable feature of LPS has been a major challenge to the pharmaceutical and medical industries. Thus, the develop- ment of a drug, which is effective in neutralizing endotoxin is urgently required. Efforts are currently underway in many labs worldwide to increase the potency and specificity of anti-LPS peptides so that they are specifically toxic to microbes but not to mammals. In our lab, we have developed Sushi 1 peptide (S1) from the LPS-sensitive protein, Factor C. We have proven by a series of approaches, that S1 peptide can effectively bind LPS and neutralize its endotoxicity. Septic shock Pyrogens are substances that cause fever when introduced intravenously. The best studied pyrogen is LPS found in the outer membrane of Gram-negative bacteria. During Gram- negative sepsis, LPS stimulates host macrophages to release inflammatory cytokines, causing inflammation which informs the host of pathogen invasion. However, excessive inflammation causes septic shock, multiple organ failure and death. In the United States, a multicentre observational cohort study has projected an estimated 751,000 cases of sepsis per annum (1). LPS structure Studies on the molecular biology of septic shock have so far focused on the chemical structure of LPS. Generally, LPS has a tripartite structure comprising three covalently linked domains: the O-specific antigen, the core oligosaccharide and lipid A (Figure 1). (a) The O-specific antigen is made up of a 1 Departments of Biological Sciences, 2 Chemistry and 3 Microbiology, National University of Singapore, Singapore 117543, Singapore; 4 Corresponding to: Dr. Jeak Ling Ding & Bow Ho, Departments of Biological Sciences and Microbiology, National University of Singapore, 14 Science Drive 4, Singapore 117543, Singapore. Tel: +65-6874-2776, Fax: +65- 6779-2486, E-mail: dbsdjl@nus.edu.sg. Received Nov 29, 2005. Accepted Feb 7, 2006. Copyright © 2006 by The Chinese Society of Immunology Abbreviations: CD, circular dichroism; LBP, LPS-binding protein; LPS, lipopolysaccharide; S1, Sushi 1 peptide; τ D the diffusion time; τ DL , the diffusion time of the large particle.