Some special properties of the super-antigen lipopolysaccharide.
Bacteria don't have it easy in their tiny Brownian world. While we have evolved large size and cleverness, they have evolved miracles of chemistry and miniaturization. One of the key classifications of bacteria is between Gram positive and negative, which refers to the Gram stain. This chemical stains the peptidoglycan layer of all bacteria, which is their "cell wall", wrapped around the cell membrane and providing structural support against osmotic pressure. For many bacteria, a heavy layer of peptidoglycan is all they have on the outside, and they stain strongly with the Gram stain. But other bacteria, like the paradigmatic E. coli, stain weakly, because they have a thin layer of peptidoglycan, outside of which is another membrane, the outer membrane (OM, whereas the inner membrane is abbreviated IM).
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| Structure of the core of LPS, not showing the further "poly" saccharide tails that would go upwards, hitched to the red sugars. At bottom are the lipid tails that form a strong membrane barrier. These, plus the blue sugar core, form the lipid-A structure that is highly antigenic. |
This outer membrane doesn't do much osmotic regulation or active nutrient trafficking, but it does face the outside world, and for that, Gram-negative bacteria have developed a peculiar membrane component called lipopolysaccharide, or LPS for short. The outer membrane is assymetric, with normal phospholipids used for the inner leaflet, and LPS used for the outside leaflet. Maintaining such assymetry is not easy, requiring special "flippases" that know which side is which and continually send the right lipid type to its correct side. LPS is totally different from other membrane lipids, using a two-sugar core to hang six lipid tails (a structure called lipid-A), which is then decorated with chains of additional sugars (the polysaccharide part) going off in the other direction, towards the outside world.
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| The long, strange trip that LPS takes to its destination. Synthesis starts on the inner leaflet of the inner membrane, at the cytoplasm of the bacterial cell. The lipid-A core is then flipped over to the outer leaflet, where extra sugar units are added, sometimes in great profusion. Then a train of proteins (Lpt-A,B,C,D,E,D) extract the enormous LPS molecule out of the inner membrane, ferry it through the periplasm, through the peptidoglycan layer, and through to the outer leaflet of the outer membrane. |
A recent paper provided the structural explanation behind one transporter, LptDE, from Neisseria gonerrhoeae. This is the protein that receives LPS from a its synthesis inside the cell, after prior transport through the inner membrane and inter-membrane space (including the peptidoglycan layer), and places LPS on the outer leaflet of the outer membrane. It is an enormous barrel, with a dynamic crack in its side where LPS can squeeze out, to the right location. It is a structure that explains neatly how directionality can be imposed on this transport, which is driven by ATP hydrolysis (by LtpB) at the inner membrane, that loads a sequence of transporters sending LPS outward.
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| Some structures of LptD (teal or red), and LPS (teal, lower) with LptE (yellow), an accessory protein that loads LPS into LptD. This structure is on the outer leaflet of the outer membrane, and releases LPS (bottom center) through its "lateral gate" into the right position to join other LPS molecules on the outer leaflet. |
LPS shields Gram-negative bacteria from outside attack, particularly from antibiotics and antimicrobial peptides. These are molecules made by all sorts of organisms, from other bacteria to ourselves. The peptides typically insert themselves into bacterial membranes, assemble into pores, and kill the cell. LPS is resistant to this kind of attack, due to its different structure from normal phospholipids that have only two lipid tails each. Additionally, the large, charged sugar decorations outside fend off large hydrophobic compounds. LPS can be (and is) altered in many additional ways by chemical modifications, changes to the sugar decorations, extra lipid attachments, etc. to fend off newly evolved attacks. Thus LPS is the result of a slow motion arms race, and differs in its detailed composition between different species of bacteria. One way that LPS can be further modified is with extra hydrophobic groups such as lipids, to allow the bacteria to clump together into biofilms. These are increasingly understood as a key mode of pathogenesis that allow bacteria to both physically stick around in very dangerous places (such as catheters), and also form a further protective shield against attack, such as by antibiotics or whatever else their host throws at them.
In any case, the lipid-A core has been staunchly retained through evolution and forms a super-antigen that organisms such as ourselves have evolved to sense at incredibly low levels. We encode a small protein, called LY96 (or MD-2), that binds the lipid-A portion of LPS very specifically at infinitesimal concentrations, complexes with cell surface receptor TLR4, and sets off alarm bells through the immune system. Indeed, this chemical was originally called "endotoxin", because cholera bacteria, even after being killed, caused an enormous and toxic immune response- a response that was later, through painstaking purification and testing, isolated to the lipid-A molecule.
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| LPS (in red) as it is bound and recognized by human protein MD-2 (LY96) and its complex partner TLR4. TLR4 is one of our key immune system alarm bells, detecting LPS at picomolar levels. |
LPS is the kind of antigen that is usually great to detect with high sensitivity- we don't even notice that our immune system has found, moved in, and resoved all sorts of minor infections. But if bacteria gain a foothold in the body and pump out a lot of this antigen, the result can be overwhelmingly different- cytokine storm, septic shock, and death. Rats and mice, for instance, have a fraction of our sensitivity to LPS, sparing them from systemic breakdown from common exposures brought on by their rather more gritty lifestyles.
