Membranes membrane-associated polysomes

Historically, endoplasmic reticulum is classified as rough or smooth, based on the presence (RER) or absence (SER) of membrane-associated polysomes 144... 

From a critical study of the metabolism of poly (A) in auxin-treated pea epicotyl, Verma and Maclachlan (73) showed that discreet classes of poly (A) (presumably part of mRNA s) are differentially associated with free and membrane-bound polysomes. The induction of specific mRNA s, the decline in the rate of synthesis of mRNA s, the polysome content per cell, and the formation of cellulase were all related to the membrane-bound polysomes. Although the rate of in vivo enzyme synthesis is... 

Verma, D. P. S./ MacLachlan, G. A. Metabolism of poly(A) in plant cells. Discrete classes associated with free and membrane-bound polysomes. Plant Physiol. 1976, 58, 405-410. 

The answer is e. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 1—40. Wilson, pp 101-120.) Protein synthesis occurs in the cytoplasm, on groups of free ribosomes called polysomes, and on ribosomes associated with membranes, termed the rough endoplasmic reticulum. However, proteins destined for secretion are only synthesized on ribosomes of the endoplasmic reticulum and are synthesized in such a manner that they end up inside the lumen of the endoplasmic reticulum. From there the secretory proteins are packaged in vesicles. The Golgi apparatus is involved in the glycosylation and packaging of macromolecules into membranes for secretion. 

The cytochrome P45o-type system not only inactivates toxic chemicals in the mammalian organism, but is Hkewise able to activate or toxify a variety of chemicals which otherwise would be harmless for mammals. Cytochrome P450 is a major integral membrane protein of both smooth and rough endoplasmic reticulum and is exclusively (> 95 %) synthesised by polysomes associated with... 

Figure 12A shows typical polysome structures released by detergent extraction of purified mitochondria prepared without the use of EDTA and with a buffer containing Mg +. These structures are to be compared with free cytoplasmic polysomes prepared from the postmitochondrial supernatant fraction without the use of detergent (Fig. 12C). That some membrane material is still associated with Triton-X-100-extract ed mitochondria-bound polysomes is suggested by the fact that rapidly sedimenting material comprising predominantly larger polysome structures appears in the gradient following treatment of the Triton-extracted preparation with deoxycholate (Fig. 12B). No similar effect is observed on the optical density profile of free cytoplasmic polysomes prepared without the use of detergent (Fig. 12C).

While it is reasonable to assume that a significant portion of the cytoplasmic ribosomes which remain attached to purified mitochondria are those visualized by electron microscopy as attached to the outer mitochondrial membrane in situ, the extent to which adventitious association of cytoplasmic polysomes with the outer mitochondrial membrane occurs during cell fractionation is difficult to evaluate quantitatively. As an approach to this problem we determined conditions which would completely eliminate polysome binding to mitochondria in vitro, and then determined what effect these conditions had on the recovery of cytoplasmic rRNA with purified mitochondria. We showed that the binding of bound 80 S polysomes or free cytoplasmic polysomes to preparations of EDTA-washed mitochondria is completely inhibited by either 350 mM KCl or 10 m aurintricarboxylic acid (ATA). We found that when cells were opened and fractionated in buffer containing either 350 mM KCl or 10 M ATA, there was no significant reduction in the recovery of cytoplasmic rRNA with mitochondria when compared to mitochondria isolated in the absence of these components. These results support the notion that ribosomes which are attached to the outer mitochondrial membrane in isolated mitochondrial preparations are those which are seen attached to the outer membrane in situ. 

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