Ribosome Membrane Attachment

Ribosomes remain attached to the ER during synthesis of signal peptide-containing proteins but are released and dissociated into their two types of subunits when the process is completed. The signal peptide is hydrolyzed by signal peptidase, located on the luminal side of the ER membrane (Figure 46-4), and then is apparently rapidly degraded by proteases. 

Several reticula of the r-ER show an ordered arrangement and many ribosomes are attached to their membrane during the primary wall development stage. As maturation proceeds, the r-ER s then gradually decrease not only in number and length of reticula, but also in the number of ribosomes. The s-ER s, on the other hand, become largest after the S3 stage, and sometimes attach ribosomes at their terminals. 

In most eukaryotic cells two major classes of ribosomes exist attached ribosomes and free ribosomes. The attached ribosomes are bound to an extensive cytoplasmic network of lipoprotein membranes called the endoplasmic reticulum. The rough endoplasmic reticulum consists of bound ribosomes the smooth endoplasmic reticulum is devoid of ribosomes. There is no structural difference between a... 

The answer is c. (Murray, pp 452—467. Scriver, pp 3—45. Sack, pp 1—40. Wilson, pp 101-120.) Prokaryotic ribosomes have a sedimentation coefficient of 70S and are composed of SOS and 30S subunits. Eukaryotic cytoplasmic ribosomes, either free or bound to the endoplasmic reticulum, are larger—60S and 40S subunits that associate to an SOS ribosome. Nuclear ribosomes are attached to the endoplasmic reticulum of the nuclear membrane. Ribosomes in chloroplasts and mitochondria of eukaryotic cells are more similar to prokaryotic ribosomes than to eukaryotic cytosolic ribosomes. Like bacterial ribosomes, chloroplast and mitochondrial ribosomes use a formylated tRNA. In addition, they are sensitive to many of the inhibitors of protein synthesis in bacteria. 

A FIGURE 16-2 Electron micrograph of ribosomes attached to the rough ER in a pancreatic acinar cell. Most of the proteins synthesized by this type of ceil are to be secreted and are formed on membrane-attached ribosomes. A few membrane-unattached (free) ribosomes are evident presumably, these are synthesizing cytosoiic or other nonsecretory proteins. [Courtesy of G. Paiade.l... 

Tantalum oxide (150 mg Ta205/m ) dust inhaled for 10 days (10 h/d) was phagocytized by rat alveolar macrophages. The cells contained agglomerations of particles of different size, all < 1 pm within phagolysosomes, unchanged mitochondria and numerous free and membrane-attached ribosomes (Nemetschek-Gansler et al. 1975). 

Fig. 2. Simplified schema for the integration of acid hydrolase biosynthesis and ixissible sites of control Acid hydrolase biogenesis would be determined and controlled genetically via mRNA coding. Full biological expression requires ribosome-imlysome formation, membrane attachment ( stereoorientation) and formation of the enzyme-synthesizing unit...

As an independent approach to determining if nascent polypeptide chains are released from attached ribosomes to the mitochondrial membrane, we labeled nascent chains as above and then determined the amount of radioactivity that could be recovered with mitochondria after removal of attached ribosomes. As shown in Table IV, when mitochondria were washed several times with buffer containing Mg" " in order to maintain the integrity of the ribosome-membrane interactions, 67% of the total incorporated radioactivity was recovered in the washed mitochondrial pellet. However, if the attached cytoplasmic ribosomes were removed by treatment with RNase and EDTA and then washed with PPi-citrate, there... 

In this regard, Hackenbrock has described an ultrastructural feature of isolated rat liver mitochondria which he has termed contact points, representing regions where the inner and outer mitochondrial membranes have come together and are possibly fused. Our studies with yeast mitochondria indicate that bound 80 S ribosomes are attached preferentially to contact points. This finding not only implies that ribosome... 

Some cell lines, such as HEK293, may detach during the permeabilization step due to a strong Ca2+ dependence for attachment. While it is critical that cells are permeabilized as a monolayer, detachment does not seem to hinder cytosolic ribosome release, as they tend to detach as a (partial) monolayer. Following the permeabilization step, cells can simply be separated from the soluble cytosol phase by centrifugation at 750-1000 Xg for 5 min. Transfer the supernatant (cytosol) to a new tube. Remove any remaining cells attached to the flask via the wash buffer, combine with the cell pellet, and recover by centrifugation. Proceed with the membrane solubilization step. 

The er is a three-dimensional membrane system (57-62). As visualized in a transmission electron microscope, there are two parallel membranes with an intervening electron transparent space, the lumen. The form and abundance of the er vary. The rer are flattened sacs with numerous attached ribosomes (15-20 nm). In contrast, the smooth er (ser) lacks ribosomes. The er seems to function as a communication system within cells and can be continuous with the outer nuclear envelope. Although the rer is involved in protein synthesis, the ser functions in glycosylation. 

Ribosomes (79-87) are small organelles 17-23 nm in diameter. They can exist in clusters known as polysomes or be attached to the er where they bind to pores in the er membrane. A major constituent of the er pore is translocon, the heterotrimetric Sec 61 protein complex. Sec 61 binds to the 80s ribosomes (86). Ribosomes consist of subunits, a 30s subunit (16srRNA and 21 proteins), and a 50s subunit (23s and 5s RNAs, > proteins and the catalytic site of peptidyl transferase). Ribosomes are the sites of protein synthesis. 

Endoplasmic Reticulum (ER) A single membrane system of two distinct types, rough and smooth. The rough ER has ribosomes attached to the membrane, whereas the smooth ER does not,... 

Replicative cycles of representative DNA and RNA viruses. A. Replicative cycle of a herpesvirus, an example of a DNA virus. 1. Attachment. 2. Membrane fusion. 3. Release of viral DNA through nuclear pores. 4. Transcription of viral mRNA. 5. Synthesis of viral proteins by host cell s ribosomes. 6. Replication of viral DNA by viral polymerases. 7. Assembly of virus particles. 

FIGURE 27-1 Ribosomes and endoplasmic reticulum. Electron micrograph and schematic drawing of a portion of a pancreatic cell, showing ribosomes attached to the outer (cytosolic) face of the endoplasmic reticulum (ER). The ribosomes are the numerous small dots bordering the parallel layers of membranes. 

BM, basement membrane ER, rough endoplasmic reticulum (with ribosomes attached smooth ER is depicted nearer the nucleus and on the right side of the cell.) DI, deep indentation of plasma membrane... 

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