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.
-The data in Table 1 suggest that the domain with the highest affinity for its ligand is:

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.
-The data in Table 1 suggest that the domain with the highest affinity for its ligand is:

Quizplus · Accepted Answer

11ecba2d_0e5c_573f_a3bf_55943d9791c7_MD0008_00 The affinity of a protein for its ligand is measured by its dissociation constant K_d, which quantifies the tendency of the protein-ligand complex to degrade into protein and ligand. Conceptually, K_d represents the concentration of ligand at which half of the proteins in the solution are bound to ligands and half are not. A small K_d indicates a low tendency for the complex to break apart and corresponds to a high affinity because only a small amount of ligand is required to achieve a high amount of binding. Table 1 shows that domain 9 binds its ligand (M6P) with a K_d of 6.6 × 10⁻⁸ mM, which is approximately half as large as the next smallest K_d of 1.1 × 10⁻⁷ mM (domain 3 binding to M6P). Therefore, domain 9 has the highest affinity for its ligand. (Choices A, B, and D) Domains 1, 5, and 11 all have K_d's for their ligands that are more than an order of magnitude greater than that of domain 9 for M6P. Therefore, they all have lower affinities for their ligands than domain 9 has for its ligand. Educational objective: The affinity of a protein for its ligand is measured by the dissociation constant K_d. A small K_d represents a low tendency of the protein-ligand complex to degrade and a high affinity of the protein for its ligand.

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