50.The answer is a. (Alberts, pp 601–602, 713, 716–719, 908–909,
923–927.)Clathrin is an important protein that forms the coating of: secretory
vesicles transported from the TGN to targets and coated pits and vesicles
involved in endocytosis. It is involved in the retrieval of membrane following
exocytosis. Spectrin heterodimers (answer b)form tetramers that interact
with actin and provide flexibility and support for the membrane. The protein
ankyrin(answer c)“anchors” the band 3 protein to the spectrin-membrane
skeleton indirectly binding band 3 protein to the cytoskeleton (spectrin
tetramers) of the red blood cell (RBC). The band 3 protein is known to be an
anion transport protein of the RBC. Actin (answer d)is the protein found in
thin filaments in the RBC cytoplasm. Intermediate filaments are important
cytoskeletal elements with specificity that depends on the origin of the cells in
question. Vimentin (answer e)is specific for cells derived from mesenchyme,
e.g., fibroblasts and chondrocytes. COP-I (COat Protein-I) coats vesicles
involved in retrograde transport from Golgi→RER(answer f).COP-II is the
coat protein for vesicles transported in an anterograde direction from the
RER→Golgi(answer g).
51.The answer is c.(Ross and Pawlina, p 44. Kasper, p 2318. Kumar, pp
160–161. Rubin, pp 254–255.)The child in the scenario suffers from Tay-Sachs
disease, a lysosomal storage disease. Lysosomes contain an array of specific
hydrolases. In Tay-Sachs disease, hexosaminidase A is deficient, resulting in
the buildup of GM2 ganglioside and leading to mental retardation, blindness,
and mortality. A pharmacological approach would target reducing GM2 gan-
glioside levels by increasing hexosaminidase A activity (answer c).Increase in
GM2 levels (answers a and b)or increased transport (answer d)to a lyso-
some deficient in hexosaminidase would worsen the disease. The table on
page 124 summarizes the enzyme deficiencies and resulting effects in some of
the more prominent lysosomal disorders. Mannose-6-phosphate and its
receptor are involved in the trafficking of proteins to the lysosomal compart-
ment. Removal of mannose 6-phosphate (answer e),as occurs in inclusion-
cell (I-cell) disease, would result in default of lysosomal enzymes to the
secretory pathway, and the hexosaminidase deficiency would worsen.
Cell Biology: Cytoplasm Answers 123