Other Outer Membrane Proteins

On contact with a mucosal surface lined by colimmar, cuboidal, or noncornifled squamous epithelial cells, the gonococci attach to cell membranes by means of surface pUi and are then pinocytosed. The virulence of the organism is mediated primarily by the presence of pili and other outer membrane proteins. After mucosal damage is established, polymorphonuclear leukocytes (PMNs) invade the tissue, submucosal abscesses form, and purulent exudates are secreted. "... 

Unlike most of the other outer membrane proteins, the lipoprotein does not serve as a binding site for any bacteriophages neither can antibodies prepar against isolated lipoprotein, be shown to bind to intact cells. These observations suggest that the proteinaceous portion of the molecule is deeply buried in the envelope. Treatment of E. coli cells with trypsin leads to a detachment of the outer membrane from the peptidoglycan. 

Mg/ml and 4 ixg/m, peak 7 protein synthesis was more resistant to chloramphenicol than the synthesis of the other outer membrane proteins. The synthesis of peak 11 protein, however, was found to be somewhat more sensitive to the drug than that of the other envelope proteins. In the case of sparsomycin, the synthesis of peak 7 protein was again found to be resistant to the drug at concentrations of 10 tig/voX and 50 Mg/ml (Table I). 

Puromycin also revealed differential inhibitory effects on the synthesis of the individual outer membrane proteins (Table I). The synthesis of peak 11 protein was found to be strikingly more resistant to puromycin than that of the other outer membrane proteins. Since puromycin is known to inhibit protein synthesis by prematurely terminating peptide formation, it is important to rule out the possibility that the appearance of peak 11 formed in the presence of puromycin may be due to the prematurely terminated peptides of other envelope proteins which have a size coincident with that of peak 11 protein and can still be incorporated into the membrane. As will be discussed in more detail later, this protein is a lipoprotein and can be specifically labeled with radioactive palmitic acid. Furthermore, since the lipoprotein does not contain any histidine residues, it can be normally synthesized even in a histidine auxotroph of E. coli which is being starved... 

As we have already seen in earlier sections, the biosynthesis of the lipoprotein has a few unique features. Its biosynthesis is extremely resistant to puromycin, and its mRNA is also extraordinarily stable. It is clear that the lipoprotein is synthesized on ribosomes, since its biosynthesis is as sensitive to chloramphenicol, tetracycline, sparsomycin, and kasugamycin as the biosynthesis of the other outer membrane proteins. 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

E. C. (1988b). Incorporation of the major outer membrane protein of Neisseria gonorrhoeae in saponin-Upid complexes (Iscoms) Chemical analysis, some structural features, and comparison of their immunogenicity with three other antigen delivery systems, Inf. Immun., 56, 432-438. 

Whether or not they are lipoproteins, both periplasmic proteins and outer membrane proteins translocate across the inner membrane thus there should be some cellular mechanisms that sort them. Unlike inner membrane proteins, outer membrane proteins do not have characteristic hydrophobic transmembrane segments as such, most, if not all, of them are thought to be composed of ft strands. Moreover, it has been suggested that such conformation may be the determinant of the integration into the outer membrane in other words, these proteins may be spontaneously integrated into the outer membrane. If this assumption is correct, the outer membrane proteins must fold at the periplasm. Another possibility is that the outer membrane proteins are integrated at certain sites where the inner and outer membranes are contacted. This issue has not been solved, but a recent experiment supports the periplasmic folding (Eppens et al., 1997). 

If the /3-rich conformation of outer membrane proteins is really the determinant of their localization, the prediction system of protein localization should evaluate the possibility of an input protein being the [3 type. Fortunately, this appears easier than ordinary secondary structure prediction of globular proteins. Several authors have proposed prediction methods. Here, a method that is conceptually simple and two other recently published methods are briefly described. 

In biochemical systems [Fe(enterobactin)]3- enters the bacterial cell through an outer membrane protein, the structure of the E. coll version of which has recently been determined.74 The catechetol functionalities are crucial to this transport but the trilactone ring can be replaced with many other... 

In an animal study, lipopolysaccarides were extracted from detoxified E. coli J5, combined with outer membrane protein of Neisseria meningitides and injected in rabbits. The serum of the immune animals protected neutropenic rats infected with other Gram-negative organisms (Bhattacharjee et al., 1996 Cross et al., 2001). 

Each tetramer comprises four closely associated monomeric channels circled by a hydrophobic surface long enough to span the lipid bilayer (Fig. 4). Toward the cytoplasmic and periplasmic surfaces are layers that include side chains of tyrosine and tryptophan that can productively interact with the polar-nonpolar interface in the lipid head-group region as in other integral membrane proteins (Koeppe and Anderson, 1996). These layers are flanked by two outer layers of charged residues, 35 A apart, that result in net positive charge on the cytoplasmic side. 

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