After isolating the rough endoplasmic reticulum from the rest of the cytoplasm, you purify the RNAs attached to it. Which of the following proteins do you expect the RNA from the rough endoplasmic reticulum to encode? a. soluble secreted proteins b. ER membrane proteins c. plasma membrane proteins d. all of the above - CORRECT ANSWER===D The rough ER consists of ER membranes and polyribosomes that are in the process of translating and translocating proteins into the ER membrane and lumen. Thus, all proteins that end up in the lysosome, Golgi apparatus, or plasma membrane, or are secreted, will be encoded by the RNAs associated with the rough ER. Cells have oligosaccharides displayed on their cell surface that are important for cellcell recognition. Your friend discovered a transmembrane glycoprotein, GP1, on a pathogenic yeast cell that is recognized by human immune cells. He decides to purify large amounts of GP1 by expressing it in bacteria. To his purified protein he then adds a branched 14-sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1 (Figure Q15-48). Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem? a. The oligosaccharide needs to be further modified before it is mature. b. The oligosaccharide should have been added one sugar at a time. c. The oligosaccharide needs a disulfide bond. d. The oligosaccharide should have been added to the serine instead of the asparagine. - CORRECT ANSWER===A Oligosaccharides are usually further modified by enzymes in the ER and the Golgi before the glycoprotein is inserted into the plasma membrane. The other choices are untrue, and thus are not good explanations. Oligosaccharides are added to the Asn and not the Ser [choice (a)] and are added as a branched 14-sugar oligosaccharide [choice (b)]. Disulfide bonding occurs between cysteines of proteins and not in sugars [choice (d)]. Figure Q15-34 shows the organization of a protein that normally resides in the plasma membrane. The boxes labeled 1 and 2 represent membrane-spanning sequences and the arrow represents a site of action of signal peptidase. Given this diagram, which of the following statements must be true? a. The C-terminus of this protein is cytoplasmic. b. The mature version of this protein will span the membrane twice, with both the N- and C-terminus in the cytoplasm. c. None of the above. d. The N-terminus of this protein is cytoplasmic. - CORRECT ANSWER===A The mature version of this protein will span the membrane once, with membranespanning segment 2 in the membrane and the C-terminus facing the cytoplasm. Plasma membrane proteins are inserted into the membrane in the . The address information for protein sorting in a eukaryotic cell is . Proteins are transported into the Golgi apparatus via . The proteins transported into the endoplasmic reticulum by a. amino acid sequence b. endoplasmic reticulum c. folded d. Golgi apparatus e. plasma membrane f. protein translocators g. sorting signal h. transport vesicles i. unfolded - CORRECT ANSWER===b, a, c, g, h, f, i In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins? a. on the rough ER b. the nucleus c. in the lumen of the ER d. in the cytosol - CORRECT ANSWER===D Ribosomes are cytoplasmic proteins and thus their protein components are translated in the cytosol. Match the components involved with ER transport with the appropriate cellular location. Locations can be used more than once, or not at all. Components: 1. signal-recognition signal 2. protein translocator 3. mRNA 4. SRP receptor