Cell Biology:
Intracellular Trafficking
Answers
54.The answer is d.(Alberts, pp 749–751.)Receptor-mediated endocyto-
sis of ligand-receptor complexes is a selective process that requires invagi-
nation of the cell membrane to form clathrin-coated pits and vesicles.
Clathrin is not involved in phagocytosis. Phagocytosis of damaged cells
occurs by evagination to engulf the IgG-coated surface of the target. Both
processes use acidification of compartments and hydrolases to uncouple
receptor and ligand (receptor-mediated endocytosis) or destroy engulfed
material (phagocytosis). Both processes use membrane-enclosed vesicles
and are associated with lysosomal activity (answers a, b, c, and e).
Low-density lipoprotein (LDL) is the form in which most cholesterol is
transported in the blood; cellular uptake of LDL is the classic example of
receptor-mediated endocytosis. The receptors are bound to clathrin-coated
pits, but the ligand is only directly bound to its cell surface receptor. The
LDL-LDL receptor (ligand-receptor) complexes are incorporated into the
cell in coated vesicles. The acidic environment of the endosome results in
the cleavage of the ligand from its receptor (i.e., LDL from its receptor). The
LDL receptors are recycled to the membrane for additional exposure to
LDL, and the LDL in the endosome is transferred to lysosomes, where it is
broken down to cholesterol.
Other ligands differ in subtle ways from LDL in their endocytic pathways.
55.The answer is c.(Alberts, pp 693–694.)The SRP prevents degradation
of newly synthesized peptides because translocation across the ER mem-
brane protects the nascent peptide from proteases. The signal hypothesis is
the basis of the targeting of transmembrane, lysosomal, and exportable
proteins across the ER membrane. It is the key event in the segregation of
noncytosolic proteins in the ER cisternae. The result of SRP activity is
attachment of ribosomes translating the secretory protein to the ER mem-
brane and the translocation of the protein across the ER membrane. The
SRP binds to the N terminus of the signal peptide (answer a)as the peptide
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