In which cellular compartment do CatB and CatL likely facilitate membrane fusion during VSV-EGP entry?

• A. Endoplasmic reticulum
• B. Golgi apparatus
• C. Endosomes
• D. Cytosol

Answer: (C)

CatB (cathepsin B) and CatL (cathepsin L) are cysteine proteases that play crucial roles in various cellular processes, including the processing of proteins and facilitating membrane fusion events. In the context of viral entry, particularly for viruses like Vesicular Stomatitis Virus (VSV) that utilize endosomal pathways for entry into host cells, these proteases become especially relevant.

The endosomal compartment serves as a critical site for the fusion of viral membranes with cellular membranes during viral entry. When VSV is internalized by the host cell, it is contained within endosomes. The acidic environment within these compartments activates certain viral glycoproteins, enabling conformational changes that promote fusion. CatB and CatL are known to facilitate this process by cleaving specific proteins, which enhances the fusion of the viral envelope with the endosomal membrane. This action is essential for the successful delivery of the viral genome into the cytosol, leading to infection.

In summary, within the virus entry pathway, the endosomes provide the appropriate biochemical environment for CatB and CatL to exert their functions related to membrane fusion. Their roles in this specific compartment highlight why the endosomal pathway is identified as the site of action for