What is the most likely pathway taken by a newly synthesized protein that will be secreted by a cell?
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Answer:
Once the ribosomes synthesizing these proteins become bound to the rough ER, the proteins enter or cross the ER membrane cotranslationally — that is, during their synthesis. Soluble proteins in this class first are localized in the ER lumen and subsequently are sorted to the lumen of other organelles or are secreted from the cell. Likewise, the integral membrane proteins in this class initially are inserted into the rough ER membrane during their synthesis; some remain there, but many eventually become localized to the plasma membrane or membranes of the smooth ER, Golgi complex, lysosomes, or endosomes.
The rough ER is an extensive interconnected series of flattened sacs, generally lying in layers (Figure 17-11). When cells are homogenized, the rough ER breaks up into small closed vesicles, termed rough microsomes, with the same orientation (ribosomes on the outside) as that found in the intact cell. The simple experiment outlined in Figure 17-12 shows that immediately after their synthesis secretory proteins are localized in the lumen of ER vesicles, although they have been synthesized on ribosomes bound to the cytosolic face of the ER membrane.
Figure 17-11. Electron micrograph of ribosomes attached to the rough ER in a pancreatic exocrine cell.
Figure 17-11
Electron micrograph of ribosomes attached to the rough ER in a pancreatic exocrine cell. Most of the proteins synthesized by this cell are to be secreted and are formed on membrane-attached ribosomes. A few membraneunattached (free) ribosomes are evident; presumably, (more...)
Figure 17-12. Experimental demonstration of location of secretory proteins just after synthesis.
Figure 17-12
Experimental demonstration of location of secretory proteins just after synthesis. Cells are incubated with radiolabeled amino acids and then are homogenized, which fractures the plasma membrane and shears the rough ER into small vesicles (microsomes). (more...)
As already noted, all of the proteins that enter the secretory pathway contain an ER signal sequence, generally at the N-terminus (see Table 17-1). This sequence directs the ribosomes that are synthesizing these proteins to the rough ER. Membrane-bound ribosomes and ribosomes free in the cytosol can be separated from other cellular constituents and from each other by a combination of differential and sucrose density-gradient centrifugation (see Figures 5-23 and 5-24). Due to the low buoyant density of phospholipids, membrane-bound ribosomes “band” at a lighter density than do free ribosomes. Biochemical analyses of purified membrane-bound and free ribosomes show that they contain exactly the same proteins and ribosomal RNAs and are functionally indistinguishable. These findings are consistent with the notion that all information for intracellular protein distribution is located in the amino acid sequence of the newly synthesized protein itself.