Human immunodeficiency virus (HIV) infects cells that have both CD4 and CCR5 cell surface molecules. The viral nucleic acid molecules are enclosed in a protein capsid, and the protein capsid is itself contained inside an envelope consisting of a lipid bilayer membrane and viral glycoproteins. One hypothesis for viral entry into cells is that binding of HIV membrane glycoproteins to CD4 and CCR5 initiates fusion of the HIV membrane with the plasma membrane, releasing the viral capsid into the cytoplasm. An alternative hypothesis is that HIV gains entry into the cell via receptor-mediated endocytosis, and membrane fusion occurs in the endocytotic vesicle. To test these alternative hypotheses for HIV entry, researchers labeled the lipids on the HIV membrane with a red fluorescent dye.

-What would be observed by live-cell fluorescence microscopy if HIV is endocytosed first, and then fuses with the endocytotic vesicle membrane?
A) A spot of red fluorescence will remain on the infected cell's plasma membrane, marking the site of membrane fusion and HIV entry.
B) The red fluorescent dye-labeled lipids will diffuse in the endocytotic vesicle membrane and become difficult to detect.
C) A spot of red fluorescence will move into the infected cell's interior.
D) A spot of red fluorescence will remain outside the cell after delivering the viral capsid.
E) Fluorescence microscopy does not have enough resolution to visualize fluorescently labeled HIV virus particles.

Question

Human immunodeficiency virus (HIV) infects cells that have both CD4 and CCR5 cell surface molecules. The viral nucleic acid molecules are enclosed in a protein capsid, and the protein capsid is itself contained inside an envelope consisting of a lipid bilayer membrane and viral glycoproteins. One hypothesis for viral entry into cells is that binding of HIV membrane glycoproteins to CD4 and CCR5 initiates fusion of the HIV membrane with the plasma membrane, releasing the viral capsid into the cytoplasm. An alternative hypothesis is that HIV gains entry into the cell via receptor-mediated endocytosis, and membrane fusion occurs in the endocytotic vesicle. To test these alternative hypotheses for HIV entry, researchers labeled the lipids on the HIV membrane with a red fluorescent dye.
 
-What would be observed by live-cell fluorescence microscopy if HIV is endocytosed first, and then fuses with the endocytotic vesicle membrane?
A) A spot of red fluorescence will remain on the infected cell's plasma membrane, marking the site of membrane fusion and HIV entry. 
B) The red fluorescent dye-labeled lipids will diffuse in the endocytotic vesicle membrane and become difficult to detect. 
C) A spot of red fluorescence will move into the infected cell's interior. 
D) A spot of red fluorescence will remain outside the cell after delivering the viral capsid. 
E) Fluorescence microscopy does not have enough resolution to visualize fluorescently labeled HIV virus particles.

Quizplus · Accepted Answer

If HIV gains entry into the cell via receptor-mediated endocytosis, the fluorescent dye-labeled lipids will be initially detectable on the HIV membrane. However, once the HIV is endocytosed, the red fluorescent dye-labeled lipids will diffuse in the endocytotic vesicle membrane and become difficult to detect. A spot of red fluorescence will move into the infected cell's interior when the endocytic vesicle fuses with a lysosome or a lipid droplet, creating an acidic environment that triggers release of the viral capsid inside the infected cell. Therefore, option C is the best choice. Option A is incorrect because there could be multiple endocytosis events, and the red fluorescent dye-labeled lipids would not be confined to a single spot. Option B is incorrect because the lipids on the HIV membrane would not be exposed to the cytoplasmic side of the endocytotic vesicle membrane. Option D is incorrect because the viral capsid is released inside the infected cell, not outside. Option E is incorrect because fluorescence microscopy has sufficient resolution to visualize fluorescently labeled HIV virus particles.

Human Immunodeficiency Virus (HIV) Infects Cells That Have Both CD4