Protein kinase is an enzyme that _____.
A) functions as a second messenger molecule
B) serves as a receptor for various signal molecules
C) activates or inactivates other proteins by adding a phosphate group to them
D) produces second messenger molecules
Use the paragraph and accompanying figure to answer the following questions.
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.