Outer Membrane

Unlike the symmetric bilayer of the inner membrane, the two leaflets of the outer membrane are different. The inner leaflet consists of familiar phospholipids. In most diderms, though, the outer leaflet is composed mainly of lipopolysaccharide, or LPS, molecules. These molecules have lipid portions which interact with the lipids of the inner leaflet and again form a barrier against hydrophilic molecules. They also have sugar portions, which extend out from the cell and present an additional barrier to hydrophobic molecules [22]. The dense LPS layer protects the cell from things like antibiotics and, for pathogens, the defenses of the host. On the flip side, our immune systems have evolved to detect and strongly react to LPS molecules.

Lipoproteins are hybrid molecules, formed from covalently-linked lipid and protein pieces. The lipid allows them to embed into a membrane, tethering the attached protein to function nearby. Braun’s lipoprotein, or Lpp [23], is one of the most abundant molecules in the outer membrane of cells like Escherichia coli. The top of the trimeric protein “coiled-coil” shown here contains the lipid tether. The bottom binds the peptidoglycan, creating a rigid link between the outer membrane and the cell wall that sets the distance between the two layers. A typical E. coli cell may contain 100,000 such links. Some other species use different proteins for the same purpose.

Compare this diderm cell wall purified from Escherichia coli to the monoderm cell wall on the last page. Since this one is thinner, we can make out more details, like the bundles of glycan strands running around the circumference of the cell. The main difference between the two types of walls seems to be whether they have largely a single layer of peptidoglycan (diderm) or many layers (monoderm). So even though the two look different, their architecture is fundamentally the same. In some circumstances, as you will see in Chapter 8, cells can even switch between the two forms.

Encasing your cell in a rigid scaffold presents a problem: how can it grow? It is easy to make membranes larger simply by adding more lipids. But to add more peptidoglycan strands, they must be linked into the existing network, which means breaking existing links to accommodate them. To do this, cells use three tools: an enzyme that links glycan sugars into strands, an enzyme that links glycan strands together with peptide bonds, and an enzyme that cuts these peptide links to allow new strands to be incorporated. Remember, though, that your cell, with its solute-rich interior, has a turgor pressure pushing outward with a force of perhaps 3 atmospheres, equivalent to what we would feel at a depth of 20 meters in the ocean. This is more than enough pressure to lyse an exposed membrane, so the tools must be wielded with care or the cell would burst. We are still figuring out how this works, with help from computer simulations like this one by Lam Nguyen. Here you see a model of an Escherichia coli cell wall being enlarged using the three enzyme tools we just described (the colored balls). This simulation was run to test whether just having the tools function in a complex rather than separately might provide enough coordination for safe growth [24]. (The answer was yes. You can watch the full movie describing this research on YouTube.)