Endoplasmic reticulum membrane-bound ribosomes

Secretory and membrane-tai eted proteins are synthesized as larger precursors on endoplasmic reticulum (ER)—bound ribosomes, which inject the protein across or into the ER membrane as it is translated. 

Surprisingly, it turned out that some of peroxisomal membrane proteins are synthesized at the ER, cotranslationally. This seems a unique example that breaks the independence between the free ribosome system and the membrane-bound ribosome system (another example is found in a sorting mechanism into the vacuole). This phenomenon may be interpreted that the peroxisome may originate from the endoplasmic reticulum, evolutionally. 

Protein processing in the endoplasmic reticulum makes mistakes. All membrane-associated proteins and proteins that are secreted by the cell are synthesised on membrane-bound ribosomes and pass into the lumen of the reticulum, where they are modified by post-translational processes, so that much biochemical manipulation of the proteins takes place. Consequentially mistakes are often made. Such abnormal proteins are exported from the lumen into the cytosol for ubiquitination and degradation in the proteasome. 

In addition to mRNA and tRNA, the third major class of RNA molecule required for protein synthesis is rRNA. Together with as many as 70 ribosomal proteins, rRNA folds into a two-subunit macromolecule complex called a ribosome (Chapter 5). In bacteria, the ribosomes attach to mRNA as it is being synthesized, thereby coupling transcription and translation. In eukaryotes, protein synthesis occurs in the cytoplasm, either by free ribosomes in the cytosol or by membrane-bound ribosomes associated with the endoplasmic reticulum. The differences between prokaryotic and eukaryotic protein synthesis are illustrated in Figure 26.3. 

The endoplasmic reticulum (ER) is folded and provides a large surface area. It is the roadway of the cell and allows for transport of materials through and out of the cell. There are two types of ER. Smooth endoplasmic reticulum contains no ribosomes on their surface. This is the site of lipid synthesis. Rough endoplasmic reticulum has ribosomes on their surface. They aid in the synthesis of proteins that are membrane bound or destined for secretion. 

Three organelles may be considered within this section the ribosome, endoplasmic reticulum and Golgi apparatus (Figure 9.2). The ribosomes may be found free in the cytoplasm or attached to membranes especially the endoplasmic reticulum (ER). Free ribosomes participate in the synthesis of intracellular proteins whilst ribosomes bound to the ER synthesize proteins destined for the cell surface as membrane components or secretions and lysosomes. The structure (Section 7.4) and function of ribosomes (Section 17.6) in either case is the same. 

One mechanism for the transfer of particular translation products across membrane barriers which has received extensive experimental support is the vectorial release of secretory proteins into the cisternal space of the endoplasmic reticulum. In this system, secretory proteins are synthesized selectively on polysomes attached to the rough endoplasmic reticulum and are subsequently released from the membrane-bound ribosomes through the adjacent membrane into the cisternum of the reticulum for later transport from the cell. ... 

We discussed earlier that one mechanism for the transfer of particular translation products across the membrane barriers is the process of vectorial translation as carried out by polysomes attached to the rough endoplasmic reticulum of secretory cells. An in vitro assay for this process has been devised by Redman and Sabatini. In order to carry out this assay, it is necessary to release labeled, nascent polypeptide chains from membrane-bound ribosomes by reaction with puromycin and assess their distribution between the medium and the membrane compartment. We followed this procedure to assay for vectorial release of nascent chains from bound 80 S polysomes attached to the outer mitochondrial membrane. 

In a series of experiments shown in Fig. 20, we determined the effect of puromycin and increasing concentrations of KCl on the release of ribosomes from purified mitochondria. With very high concentrations of KCl, about one-third of the attached ribosomes are removed. However, the removal of virtually all the membrane-bound ribosomes is affected only by the combined action of puromycin and high concentrations of KCl. These results show that, like the attachment of ribosomes to the rough endoplasmic reticulum of secretory cells, the attachment of cytoplasmic... 

These questions must be examined with regard to local circumstances of biosynthesis. In cells, proteins are synthesized either by free polysomes or by rough endoplasmic reticulum (RER) membrane-bound polysomes. For nascent polypeptide chains, these two different situations have to be considered since, in the second case, compartmentalization leads to a vectorial transport and discharge through the membrane. Bound and free polysomes seem to derive from a common precursor pool of ribosomes. Several lines of evidence indicate that membrane-bound ribosomal subunits readily exchange in vivo and no major differences in the rRNA sequences and organization of free and bound ribosomes have been found (Fern and Garlik, 1976 see Shore and Tata, 1977). 

The endoplasmic reticulum (ER) is an extensive closed membrane system consisting of tubular and saccular structures. In the area of the nucleus, the ER turns into the external nuclear membrane. Morphologically, a distinction is made between the rough ER (rER) and the smooth ER (sER). Large numbers of ribosomes are found on the membranes of the rER, which are lacking on the sER. On the other hand, the sER is rich in membrane-bound enzymes, which catalyze partial reactions in the lipid metabolism as well as biotransformations. 

Translating ribosomes in eukaryotes are located in different places in the cell depending on the fate of their proteins. Free polysomes are in the cytoplasm and synthesize cytoplasmic proteins and those that are bound for most intracellular organelles, for example, the nucleus. Members of the second class of polysomes, membrane-bound polysomes, are attached to the endoplasmic reticulum (forming the rough ER), and synthesize exported proteins. In cells that are actively secreting enzymes or hormones (for example, those in the pancreas), most of the protein synthesis occurs on the rough ER. 

The reactions of phase 2 relate to the attachment of the bridge-carbohydrate residues to the polypeptide chain. There is evidence showing that this addition occurs while the polypeptide chain is still attached to, or perhaps still being synthesized on, the ribosomes.101-103 Thus, 14C-labeled 2-amino-2-deoxy-D-glucose, injected into the circulatory system of the rat, was incorporated into protein in the ribosomes of the rough endoplasmic-reticulum of the liver. Administration of puromycin caused release of the 14C-labeled glycoprotein, which could be isolated by acid-precipitation methods. Examination of the radioactivity data revealed that the subcellular structures most actively involved in glycoprotein synthesis were the ribosomes bound to the membrane, and not free polysomes. 

The rest of the known sequences of all four classes of BCB domain-containing proteins feature signal peptides in their precursors, indicating that they are translocated across the bacterial cytoplasmic membrane or are translated on the endoplasmic reticulum-bound ribosomes and sent to the secretory pathway in eukaryotes. Thus they are located in the bacterial periplasm, secreted into the extracellular milieu, or anchored to the cell surface. 

The amino acids of a protein control its location in the cell. Some proteins are water soluble, whereas others are bound to the ceil membrane (plasma membrane), the mitochondrial membrane, and the membranes of the endoplasmic reticulum and nucleus. The association of a protein with a membrane is maintained by a stretch of lipophilic amino acids. Insertion of this stretch into the membrane occurs as the protein is synthesized. Water-soluble proteins are formed on ribosomes that "float" free in the cytoplasm. Membrane-bound proteins are formed on ribosomes that associate with the endoplasmic reticulum (ER). As the amino acids are polymerized in the vicinity of the F,R, a stretch of lipophilic acids becomes inserted into the membrane of the FR. This anchoring of the protein is maintained when it is shuttled from its location in the ER to its desired location in the plasma membrane. 

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