Passage
Many lysosomal proteins are transported to the lysosome by the cation-independent mannose 6-phosphate receptor (CI-MPR). CI-MPR is a transmembrane protein found in the Golgi apparatus and vesicles. It contains 15 globular domains folded similarly on the luminal side of the membrane, each of which has up to 3 glycosylation sites and 6−12 cysteine residues that aid in folding.The ability of CI-MPR to recognize multiple different ligands was studied by fluorescent microarray analysis. Microarray analysis was performed by cross-linking candidate ligands such as plasminogen, insulin-like growth factor 2 (IGF-II), and mannose 6-phosphate (M6P), to glass slides. The slides were then incubated with full-length CI-MPR or recombinantly expressed CI-MPR domains, followed by incubation with an anti−CI-MPR antibody. A fluorescently labeled antibody that binds the first antibody was then added, and the relative fluorescence levels of each candidate ligand were compared. Higher fluorescence levels correspond to greater binding.Experiment 1Microarray analysis was carried out on slides containing varying levels of candidate ligands, and binding affinities of each CI-MPR domain for its primary ligand were estimated. Table 1 shows the results.Experiment 2Domains 3, 5, and 9 bind carbohydrates. Amino acids in these domains were mutated to determine which are required for binding. Based on this experiment, scientists concluded that Arg, Glu, and Tyr are required for carbohydrate binding by domains 3, 5, and 9 of CI-MPR.Table 1
Adapted from Bohnsack RN, Song X, Olson LJ, et al. Cation-independent mannose 6-phosphate receptor: a composite of distinct phosphomannosyl binding sites. J Biol Chem. 2009;284(50):35215-26.
-Which of the following is a covalent bond that could stabilize the tertiary structure of CI-MPR domains?

Question

Passage
Many lysosomal proteins are transported to the lysosome by the cation-independent mannose 6-phosphate receptor (CI-MPR).  CI-MPR is a transmembrane protein found in the Golgi apparatus and vesicles.  It contains 15 globular domains folded similarly on the luminal side of the membrane, each of which has up to 3 glycosylation sites and 6−12 cysteine residues that aid in folding.The ability of CI-MPR to recognize multiple different ligands was studied by fluorescent microarray analysis.  Microarray analysis was performed by cross-linking candidate ligands such as plasminogen, insulin-like growth factor 2 (IGF-II), and mannose 6-phosphate (M6P), to glass slides.  The slides were then incubated with full-length CI-MPR or recombinantly expressed CI-MPR domains, followed by incubation with an anti−CI-MPR antibody.  A fluorescently labeled antibody that binds the first antibody was then added, and the relative fluorescence levels of each candidate ligand were compared.  Higher fluorescence levels correspond to greater binding.Experiment 1Microarray analysis was carried out on slides containing varying levels of candidate ligands, and binding affinities of each CI-MPR domain for its primary ligand were estimated.  Table 1 shows the results.Experiment 2Domains 3, 5, and 9 bind carbohydrates.  Amino acids in these domains were mutated to determine which are required for binding.  Based on this experiment, scientists concluded that Arg, Glu, and Tyr are required for carbohydrate binding by domains 3, 5, and 9 of CI-MPR.Table 1
  Adapted from Bohnsack RN, Song X, Olson LJ, et al. Cation-independent mannose 6-phosphate receptor: a composite of distinct phosphomannosyl binding sites. J Biol Chem. 2009;284(50):35215-26.
-Which of the following is a covalent bond that could stabilize the tertiary structure of CI-MPR domains?

Quizplus · Accepted Answer

11ecba2d_0e5a_5b6d_a3bf_7f57fe46b553_MD0008_00 A protein's tertiary structure is stabilized by interactions between amino acid side chains. Most of these interactions are noncovalent, including hydrogen bonding and electrostatic attractions. In oxidizing environments such as the lumen of vesicles and the extracellular space, covalent bonds can form between cysteine residues. Because these bonds form between two sulfur atoms, they are known as disulfide bonds. The passage states that each domain of CI-MPR has between 6 and 12 cysteine residues. Because the globular domains are found in the oxidizing environment of the Golgi apparatus and its vesicles, their tertiary structure is likely stabilized by disulfide bonds between these cysteine residues. (Choice A) Peptide bonds are covalent bonds that form between amino acids to make proteins and peptides. They contribute to a protein's primary structure but not its tertiary structure. (Choice C) Electrostatic interactions are frequently involved in tertiary protein structure, but they do not involve shared electrons and therefore are not covalent bonds. (Choice D) Thioester bonds form between a sulfur atom and a carbonyl carbon. They are important for coenzymes and metabolites such as acetyl-CoA and succinyl-CoA, but they do not participate in tertiary protein structure. Educational objective: Tertiary protein structure is stabilized by interactions between amino acid side chains. Most of these interactions are noncovalent, but covalent disulfide bonds may form between cysteine residues in oxidizing environments such as the lumen of a vesicle or the extracellular space.

Passage Many Lysosomal Proteins Are Transported to the Lysosome by the the Cation-Independent