Lipopolysaccharide, interaction with outer membrane proteins

The mode of interaction of the ordered hexagonal lattice structure with a lattice constant of about 7 nm, which can be reconstituted from the outer membrane protein 0-8 and the lipopolysaccharide of Escherichia coli K-12 has been studied by electron microscopy. " Both the fatty acid region and the polysaccharide region of the lipopolysaccharide are involved in an interaction of lipopolysaccharide with the membrane protein. 

Because of the limited number of functions of the outer membrane, it consists of much fewer protein components than does the cytoplasmic membrane.As will be discussed later, the major outer membrane proteins exist in extremely large quantities, of the order of 1-8 x 10 molecules/cell, suggesting that they occupy a considerable part of the outer membrane structure. This unique feature of the outer membrane provides us with an excellent system for the investigation of the mechanism of biosynthesis and the mode of assembly of individual outer membrane proteins, their interaction with other proteins, phospholipids, and lipopolysaccharide, and their functions. 

All tailed phages have evolved tailspike and fiber proteins for efficient virus-host-interactions. These specialized adhesions mediate the recognition and attachment to the bacterial surface and constitute the key determinants for host specificity. Interestingly, many spikes and fibers are composed of homotrimeric complexes which remain stable even in the presence of sodium dodecyl sulfate (SDS) [12, 14, 30-34], Several phages have developed tailspike proteins with an enzymatic activity in order to penetrate the thick layer of lipopolysaccharides or capsular polysaccharides of many pathogenic bacteria. These capsule-specific depolymerases (hydrolases or lyases) are required to gain access to and to fix the phage at the bacterial outer membrane [13, 14, 35-38]. 

The next barrier present in gram-negative bacteria is also a continuous, but a much more compact, layer of closely packed macromolecules. This entity, referred to as the outer membrane, consists predominantly of lipopolysaccharides (LPS), proteins, and phospholipids. The proteins have been estimated to contribute to about 50% of the surface area of this layer. The lipid-protein-polysaccharide components of the outer membrane form a matrix which appears in cross section as a typical double-tracked structure, similar to a phospholipid bilayer. This matrix is involved in a multiplicity of interactions of the cell surface with its exterior and interior environment. The outer membrane is known to function as a barrier to a large number of compounds, such as the antibiotics, actinomycin D, penicillin G, polymyxin, and bacitracin. In addition to preventing the entry of certain compounds, the outer membrane can also be viewed as a means of keeping enzymes of the periplasmic space trapped within the domain of the cell. 

It is now evident that divalent cations play an important role in the interaction of the lipopolysaccharide, phospholipid, and protein components of the outer membrane. When intact cells are treated with EDTA, as much as 90% of the LPS, 5-10% of the protein, and 5% of the phospholipid of the membrane are released. There is also some evidence that the remaining LPS represents a fraction which is newly synthesized... 

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