Membranes, lipoprotein, translocation

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). 

Fig. 14. Cotranslational assembly of a lipoprotein from the inner leaflet of the ER biiayer and apoB. In this model, translation of the C-terminal portion of apoB proceeds on a membrane-bound ribosome, while translocation and lipoprotein assembly occur on the luminal side of the ER. The N-terminal portion of apoB is believed to be embedded in the inner leaflet of the bilayer, where it nucleates the formation of an oil droplet from the supersaturated ER membranes. As the hydrophobic inner surface of apoB attempts to surround the oil droplet, it bulges into the lumen, as depicted here. On completion of translation, the two ends of apoB become free to meet, which would automatically result in the detachment of the lipoprotein from the bilayer. (Not drawn to scale.)...

Both simple and complex species (e.g., water, lipids, proteins, lipoproteins, antibodies, enzymes, viruses, and small and large electrolyte ions) may be adsorbed on or transported across the interfaces of biological membranes, membrane subunits, and particles. These translocations are associated with membrane or particle formation, fusion, exchange, and lysis. The processes may be relevant to the movement, exchange, and clearance of extracellular material at the alveolar lining layer of the lung (7). Within this framework, the role of surface and hypophase viscosity has not been investigated. 

Gillece P, Pilon M, Romisch K (2000) The protein translocation channel mediates glycopeptide export across the endoplasmic reticulum membrane. Proc Natl Acad Sci USA 97 4609 -4614 Ginsberg HN (1997) Role of lipid synthesis, chaperone proteins and proteasomes in the assembly and secretion of apoprotein B-containing lipoproteins from cultured liver cells. Clin Exp Pharmacol Physiol 24 A29-A32... 

In addition to being found in vertebrates and insects, lipoproteins are also found in bacteria [62]. Bacterial lipoproteins are a set of membrane proteins with distinct functions, such as adhesion to host cells, modulation of inflammatory processes, and translocation of virulence factors into host cells. These properties of bacterial lipoproteins may be utilized for development of strong vaccine adjuvants. 

Figure 3.31 Post-translational Modification of the Pro-lipoprotein in coH. An extracellular protease cleaves off the 20 amino acid signal sequence necessary for translocation across the cytoplasmic membrane, revealing the A/-termlnal cysteine. This is modified and acylated to give the free-llpoprotein, which may subsequently bind to the peptidoglycan...

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