If you presented two substances–(1) LDL particles and (2) large aggregates of 'foreign' macromolecules–to a mixture of cells, which uptake mechanism would you expect the cells to use to ingest each substance? What distinguishes the uptake mechanisms?

Question

What will be an ideal response?

Answer 1

Answer:
(1) LDL: Cells with LDL receptors would use receptor-mediated endocytosis (RME) internalization of the LDL receptor to capture and internalize LDL in clathrin-coated vesicles. LDL would be delivered from endosomes to lysosomes for degradation.
(2) Macromolecule aggregates: Certain types of immune system cells (neutrophils and macrophages) would be expected to internalize foreign macromolecules by phagocytosis and deliver them to a lysosome for destruction.
Chapter Section: 12.5
Bloom's Taxonomy: Knowledge
Learning Outcome: 12.1
Global LO: G2

2) Design an experiment that will allow you to follow a protein from its synthesis to constitutive secretion from the cell. Where would you expect to find the protein at different times during the process? How would the pathway differ if the protein obtained a mannose-6-phosphate modification during transit?
Answer: The newly synthesized protein must be labeled in some way to follow its progress through the cell. The protein could be labeled radioactively, as in the Jamieson and Palade pulse label experiment, or it could be expressed as a chimeric GFP protein. A radioactive protein can be followed with autoradiography. A GFP protein can be visualized with fluorescence microscopy. The protein would follow a path from the rER through the Golgi into secretory vesicles that release the protein out of the cell when they fuse with the plasma membrane. A protein with an M-6-P modification would be sorted from the Golgi into vesicles that deliver it to lysosomes.
Chapter Section: 12.4
Bloom's Taxonomy: Synthesis
Learning Outcome: 12.1
Global LO: G1, G8
3) Explain the mechanism by which the fungal toxin brefeldin A affects retrieval of KDEL-tagged proteins from the cis-Golgi network back to the endoplasmic reticulum.
Answer: KDEL-tagged proteins are bound by receptors in the CGN and incorporated into COPI-coated vesicle for retrograde transport back to the ER. COPI coat formation is initiated by activation of an ARF GTPase protein by a guanine nucleotide exchange factor (GEF) associated with the CGN membrane. The GEF stimulates ARF loss of bound GDP and binding of GTP (nucleotide exchange), which exposes an ARF hydrophobic N-terminal region that binds the ARF to the membrane. ARF binding recruits other COPI coat components to form the coat and initiate the retrograde transport. Brefeldin A inhibits GEF activation of ARF, thereby blocking COPI vesicle formation and retrieval of KDEL-tagged proteins to the ER.