Cell membranes, assembly proteins

Although the whole-cell membrane is non-conductive, there are several redox proteins anchored on/in the membrane that confer nano-scale conductivity to the membrane and directly enable electron transfer across the cell membrane. These proteins usually assemble together in the periplasm and/or on/across the outer-surface membrane and act as an electron transfer chain to relay the electron across the membrane. For example, the membrane-bound electron transfer chain of Shewanella oneidensis is a trans icosa-heme complex, MtrCAB, that can move electrons across the membrane. The MtrC is a decaheme cytochrome located on the outside of the outer cell membrane that mediates the electron transfer to the extracellular substrate e.g, solid electrode). MtrAB is the transmembrane electron transfer module that is responsible for electron transport from the periplasm to MtrC. More interestingly, recent findings indicates that this electron conduit is capable of reverse electron transfer, ie., electron up-take from extracellular electrodes. ... 

Virus maturation and assembly at the cell membrane or the nuclear membrane has long been seen as a potential target for antiviral compounds. For the virus to mature and be released in a conformation that will insure stability and survival of the viral genome in the exttacellular enviromnent, the protein subunits of the capsid or nucle-ocapsids have to be transported to the assembly point where they will form the final particles around the viral nucleic acid. If this process does not occur in an orderly and programmed manner, the capsid subunits will not form the required multimers and the viral components will become targets for the cellular disposal mechanisms. 

There are many cellular membranes, each with its own specific features. No satisfactory scheme describing the assembly of any one of these membranes is available. How various proteins are initially inserted into the membrane of the ER has been discussed above. The transport of proteins, including membrane proteins, to various parts of the cell inside vesicles has also been described. Some general points about membrane assembly remain to be addressed. 

Figure 46-8. Fusion of a vesicle with the plasma membrane preserves the orientation of any integral proteins embedded in the vesicle bilayer. Initially, the amino terminal of the protein faces the lumen, or inner cavity, of such a vesicle. After fusion, the amino terminal is on the exterior surface of the plasma membrane. That the orientation of the protein has not been reversed can be perceived by noting that the other end of the molecule, the carboxyl terminal, is always immersed in the cytoplasm. The lumen of a vesicle and the outside of the cell are topologically equivalent. (Re drawn and modified, with permission, from Lodish HF, Rothman JE The assembly of cell membranes. Sci Am [Jan] 1979 240 43.)...

All enveloped human vimses acquire their phospholipid coating by budding through cellular membranes. The maturation and release of enveloped influenza particles is illustrated in Fig. 3.8. The capsid protein subunits are transported flom the ribosomes to the nucleus, where they combine with new viral RNA molecules and are assembled into the helical capsids. The haemagglutinin and neuraminidase proteins that project fiom the envelope of the normal particles migrate to the cytoplasmic membrane where they displace the normal cell membrane proteins. The assembled nucleocapsids finally pass out from the nucleus, and as they impinge on the altered cytoplasmic membrane they cause it to bulge and bud off completed enveloped particles flxm the cell. Vims particles are released in this way over a period of hours before the cell eventually dies. 

The hypothesis of the participation of those cholesterol transporters (NPCILI and ABCAl) in the carotenoid transport remains to be confirmed, especially at the in vivo human scale. If the mechanism by which carotenoids are transported through the intestinal epithelial membrane seems better understood, the mechanism of intracellular carotenoid transport is yet to be elucidated. The fatty acid binding protein (FABP) responsible for the intracellular transport of fatty acids was proposed earlier as a potential transporter for carotenoids. FABP would transport carotenoids from the epithelial cell membrane to the intracellular organelles such as the Golgi apparatus where CMs are formed and assembled, but no data have illustrated this hypothesis yet. 

Many aspects of DNA replication in filamentous phages are similar to that of < >X 174. The unique property, release without cell killing, can be briefly discussed. The release from the cell occurs by a budding process in which the virus particle is always released from the cell with the end containing the A protein first. Interestingly, the orientation of the virus particle across the cell membrane is the same for its entry and exit from the cell. There is no accumulation of intracellular virus particles the assembly of mature virus particles occurs on the inner cell membrane and virus assembly is coupled with the budding process. 

In the absence of steroid hormones the receptors remain in an inactive complex, designated the apo-receptor complex (review Pratt, 1993 Bohen, 1995). In the apore-ceptor complex the receptor is boimd to proteins belonging to the chaperone class. Chaperones are proteins whose levels are increased as a result of a stress situation, such as a rise in ambient temperature. The chaperones assume a central function in the folding process of proteins in the cell. Chaperones aid proteins in avoiding incorrectly folded states. They participate in the folding of proteins during and after ribosomal protein biosynthesis, during membrane transport of proteins, as well as in the correct assembly of protein complexes. 

The function of many adaptor proteins is closely linked with the cell membrane or with the cytoskeleton. The occurrence of PH domains and myristoyl modifications suggests that adaptor proteins are involved in particular in coordination and assembly of signal complexes on the iimer side of the ceU membrane. 

Viral replication consists of several steps (Figure 49-1) (1) attachment of the vims to receptors on the host cell surface (2) entry of the virus through the host cell membrane (3) uncoating of viral nucleic acid (4) synthesis of early regulatory proteins, eg, nucleic acid polymerases (5) synthesis of new viral RNA or DNA (6) synthesis of late, structural proteins (7) assembly (maturation) of viral particles and (8) release from the cell. Antiviral agents can potentially target any of these steps. 

Musil LS, Goodenough DA Multisubunit assembly of an integral plasma membrane channel protein, gap junction connexin43, occurs after exit from the ER. Cell 1993 74 1065-1077. 

The end result of integration is the incorporation of the viral DNA into the DNA of the host cell. Once there, the provirus can serve as a template for the production of mRNA, allowing for the synthesis of viral proteins. These are assembled at the cell membrane to produce new viral particles, which then bud off to seek out new cells to infect. The integrated viral DNA is also necessarily copied whenever the host cell undergoes cell division. The insidious nature of... 

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