Protein assembly into cell membranes

In this section we describe briefly the two models for inserting integral proteins into cell membranes with special emphasis on the protease(s)-catalyzed hydrolytic modification of these proteins associated with the membrane assembly process. These two models are (1) self assembly following translation of the proteins and (2) coupling of translation with insertion of the protein into the membrane. 

Assembly of Proteins into Cell Membranes. The individual components of a biological membrane, after separation in the presence of a high concentration of detergent, still are able to associate into a membrane when the detergent is removed. However, such self-assembly of the normal constituents of a membrane fail to show asymmetry. This might be the result of a random insertion of integral proteins as the lipid... 

Gap junctions are the sites of intercellular membrane channels which provide for direct cytoplasmic continuity between the adjacent cells (Simon and Goodenough, 1998). A wealth of recent data have indicated that connexins are the proteins assembled into gap junction channels, and represent the building blocks of these channels (see the reviews of Bennett, 1997 Hormuzdi et al., 2004). 

Fig. 14.22. Infection of a host cell by HIV. The HIV virus particle binds to the CD4 receptor and a chemokine coreceptor in the host cell membrane. The virus enters the ceU and uncoats, releasing its RNA and proteins. The viral enzyme reverse transcriptase produces a double-stranded DNA copy that is integrated into the host cell genome. HIV is now a provirus. Transcripts of the viral DNA are sphced and translated to produce the proteins Tat, Rev, and Nef. Tat stimulates transcription of the viral DNA, and Rev causes the viral RNA transcripts to leave the nucleus unsphced. The unspliced RNA serves as the viral genome and also codes for the proteins of the viral core and envelope. The envelope proteins (gp41 and gpl20, which are derived from the env protein) enter the cell membrane. The viral core proteins are S5mthesized as a polyprotein, which is cleaved by a protease as the viral particles form and bud from the ceU membrane. The particles carry membrane lipid as a coat that contains gp41 and gpl20. Nef indirectly aids in the assembly of viral particles. Pol is the reverse transcriptase produced from the viral RNA. = stimulates.

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. 

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. 

Despite the weakness and short-range nature of protein-lipid and lipid-lipid interactions, cells have nevertheless evolved means of laterally assembling into membrane-mi-crodomains. Sphingolipid-cholesterol rafts serve to recmit a specific set of membrane proteins and exclude others [24]. Caveolae are deeply invaginated raft domains that are stabilized by caveolin protein oligomers (binding cholesterol) [25]. 

From these experiments it is not possible to find out whether the El protein has its separate signal peptide, mainly because of the low yields of the El protein in the synchronized translation experiments. However, evidence for such a peptide has come from studies with a temperature-sensitive mutant of SFV in which the cleavage between the capsid and the p62 protein is blocked (Hashimoto etal., 1981). In cells infected with this mutant at the restrictive temperature, the El protein is assembled in the correct orientation into the membrane of the ER. The uncleaved protein containing the capsid and the p62 sequences (Mr = 87,000) is left in the cytoplasm. These findings suggest that the El protein has its own signal peptide which might be located in the 6K protein. 

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... 

FIGURE 11-8 Integral membrane proteins. For known proteins of the plasma membrane, the spatial relationships of protein domains to the lipid bilayer fall into six categories. Types I and II have only one transmembrane helix the amino-terminal domain is outside the cell in type I proteins and inside in type II. Type III proteins have multiple transmembrane helices in a single polypeptide. In type IV proteins, transmembrane domains of several different polypeptides assemble to form a channel through the membrane. Type V proteins are held to the bilayer primarily by covalently linked lipids (see Fig. 11-14), and type VI proteins have both transmembrane helices and lipid (GPI) anchors. 

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