1. Fibroblast cells from patients W, X, Y. and Z, each of whom has a different i
ID: 40154 • Letter: 1
Question
1. Fibroblast cells from patients W, X, Y. and Z, each of whom has a different inherited defect, all contain inclusion bodies, which are lysosomes titled with undigested material. You wish to identify the cellular basis of these defects. The possibilities are: 1. a defect in one of the lysosomal hydrolases 2. a defect in the phosphotransferase that is required for mannose-6-phosphaic tagging of the lysosomal hydrolases 3. a defect in the mannose-6-phosphate receptor When you incubate some of these mutant fibroblasts in a medium in which normal cells have been grown, you find that the inclusion bodies disappear. Because of these results, you suspect that the constitutive exocytic pathway in normal cells is secreting lysosomal hydrolases that are being taken up by the mutant cells. (It is known that some mannose-6-phosphate receptor molecules arc found in the plasma membrane.) You incubate cells from each patient with medium from normal cells and medium from each of the other mutant cell cultures. and get the results summarized in the Table.Explanation / Answer
Patient W: has a defect in the mannose-6-phosphate receptor. Due to this, the cell fails to transport the hydrolases to lysosome, though the enzymes are synthesized normally. This is why the cells fail to uptake hydrolases secreted into the medium, by other cells.
Patient X: Has a defect in phosphotransferase system. The defective transferase inhibited the enzymes from being transported into the lysosomal compartment from the Cis-golgi apparatus to the trans-golgi apparatus. However, these cells can take up secreted lysosomes from other cells and function normally.
Patients Y and Z: Defect in one of the lysosomal hydrolases. These two cells complement the normal hydrolases from one another, and from those of the normal cells.
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