Team:Fatih Turkey/LALF

Lipopolysaccharide (LPS), or endotoxin, is the major mediator of septic shock, a serious complication of Gram-negative bacterial infections in humans. Molecules that bind LPS and neutralize its biological effects or enhance its clearance could have important clinical applications. Limulus anti-LPS factor (LALF) binds LPS tightly, and, in animal models, reduces mortality when administered before or after LPS challenge or bacterial infection. The wedge-shaped molecule has a striking charge distribution and amphipathicity that suggest how it can insert into membranes. The binding site for LPS probably involves an extended amphipathic loop, and it has been proposed that two mammalian LPS-binding proteins will have a similar loop. The amphipathic loop structure may be used in the design of molecules with therapeutic properties against septic shock.

Horseshoe crabs (Limulus polyphemus and Tachypleus tridentatus) are ancient arachnids that possess a primitive circulatory system, the hemolymph, containing only one kind of cell, the hemocyte. Exposure of hemocytes to bacterial endotoxins [lipopolysaccharide (LPS)] results in the activation of an intracellular coagulation cascade (Iwanaga et al., 1986), a defense against microbial invasion. The system consists of several proteins, including one that may inhibit the cascade, called anti-LPS factor(Morita et al., 1985). Limulus anti-LPS factor (LALF) is a small (101 amino acids), basic protein (Aketagawa et al., 1986; Muta et al., 1987), which binds and neutralizes LPS (Wainwright et al., 1990) and has a strong anti-bacterial effect on the growth of Gram-negative R-type bacteria (Morita et al., 1985)(22). Our interest in determining the crystal structure of LALF arose from its potential in designing molecules that would have therapeutic properties in humans. It has been proposed that LALF has sequence similarity with ct-lactalbumin, a protein that binds LPS in vitro (Aketagawa et al., 1986).

LALF recognizes the lipid A portion of individual soluble LPS molecules (Warren et al., 1992), which are obtained below the critical micellar concentration. The simplest molecules that bind lipid A with high affinity are the polymyxin family of antibiotics; these are positively charged amphipathic cyclic oligopeptides linked to a single fatty acid (Morrison and Jacobs, 1976).