Which of the following is TRUE of material entering the cell through a clathrin-coated pit during receptor-mediated endocytosis? A. Receptors and ligands are always returned to the plasma membrane. B. Receptor and ligand are internalized and delivered to the cis-Golgi for sorting. C. Receptor and ligand are released fromendosomesinto the cytoplasm immediately after internalization. D. Receptors can become ubiquitinated, targeted to multivesicular bodies, and degraded by the lysosome. E. All of the above are false statements.
The correct answer and explanation is:
The correct answer is D. Receptors can become ubiquitinated, targeted to multivesicular bodies, and degraded by the lysosome.
Explanation:
In receptor-mediated endocytosis, the process begins when specific ligands bind to their corresponding receptors on the plasma membrane, forming a clathrin-coated pit. This pit then buds off from the plasma membrane and forms a vesicle that is internalized into the cell. Once inside, the vesicle uncoats and fuses with early endosomes. At this point, different fates are possible for both the receptor and its ligand.
Option A suggests that receptors and ligands are always returned to the plasma membrane, but this is not always true. In many cases, receptors are recycled back to the membrane, but some can be targeted for degradation depending on the signaling or sorting mechanisms in place. The ligands, however, are usually delivered to lysosomes or endosomes for processing.
Option B is incorrect because while the receptor-ligand complex can be sorted within early endosomes, it is not typically delivered to the cis-Golgi for sorting. The Golgi is involved in the biosynthetic pathway, not in the sorting of endocytosed material.
Option C is also inaccurate. After internalization, the receptor-ligand complex is not immediately released into the cytoplasm. Instead, it is usually delivered to endosomes where the acidic environment helps dissociate the ligand from the receptor. The receptor may then be recycled back to the membrane, and the ligand is typically targeted for degradation in the lysosome.
Option D is correct because the receptors can be ubiquitinated, which marks them for trafficking to multivesicular bodies (MVBs). These MVBs eventually fuse with lysosomes, leading to the degradation of the receptor. This process plays an essential role in regulating the number of receptors at the plasma membrane and controlling signal transduction.
Thus, D is the most accurate statement about clathrin-mediated endocytosis.