Question

8. a) Which of the proteins from the secretory pathway could result in an enlarged Rough ER when mutated?

b) What phenotype will you expect for a mutation that abolishes the hydrolysis of GTP in the Sar protein?

c) What changed would you expect on the structure of the rough ER upon addition of a drug that interferes with the ability of ribosomes to bind to mRNA?

Answer 1

ANSWERS

8. (a) Many important experiments on the secretory pathway take advantage of cells that are specialized for the secretion of specific proteins . These cells contain organelles such as the rough ER and Golgi cisternae in abundance. For example, of the total protein made by hepatocytes (the principal cells of the liver), about 70 percent consists of proteins, such as albumin and transferrin, that are secreted into the blood. Likewise, pancreatic acinar cells synthesize several digestive enzymes that are packaged into zymogen vesicles and secreted into ductules that lead to the intestine . All cells, however, secrete some proteins. Extracellular matrix proteins such as collagens, proteoglycans, and fibronectin, for example, constitute about 5 percent of the protein made by most cultured cells. All eukaryotic cells use essentially the same pathway for synthesis and sorting of secretory proteins.

(b)these mutant cells exhibit normal rates of secretion and normal membrane morphology as determined by electron microscopy. Furthermore, the  allele does not exhibit dominant phenotypes in cell growth and secretion when overexpressed. Together, these results lead us to suggest that, contrary to current models for function, GTP hydrolysis is not essential either for vesicular transport or as a timer to turn off membrane fusion but only for recycling of   compartments. Finally, the  allele, like the wild type, is inhibited by dominant nucleotide-free  mutations. Such mutations are thought to exert their dominant phenotype by sequestration of the guanine nucleotide exchange factor (GNEF). These results suggest that the function of allel in vesicular transport requires not only the GTP-bound form of the protein but also the interaction of allel with its GNEF.

(c)In mammalian cells, the import of proteins into the ER begins before the polypeptide chain is completely synthesized—that is, import is a co-translational process. This distinguishes the process from the import of proteins into mitochondria, chloroplasts, nuclei, and peroxisomes, which are posttranslational processes. Since one end of the protein is usually translocated into the ER as the rest of the polypeptide chain is being made, the protein is never released into the cytosol and therefore is never in danger of folding up before reaching the translocator in the ER membrane. Thus, in contrast to the posttranslational import of proteins into mitochondria and chloroplasts, chaperone proteins are not required to keep the protein unfolded. The ribosome that is synthesizing the protein is directly attached to the ER membrane. These membrane-bound ribosomes coat the surface of the ER, creating regions termed rough endoplasmic reticulum, or rough ER .