Synthesis and quality control of viral membrane proteins.

Viruses use the host cellular machinery to translate viral proteins. Similar to cellular proteins directed to the secretory pathway, viral (glyco)proteins are synthesized on polyribosomes and targeted to the endoplasmic reticulum (ER). For viruses that encode polyproteins, folding of the individual proteins of the precursor often is coordinated. Translocation and the start of folding coincide and are assisted by cellular folding factors present in the lumen of the ER. The protein concentration a newborn protein finds in this compartment is enormous (hundreds of mg/ml) and the action of molecular chaperones is essential to prevent aggregation. Viral envelope proteins also undergo the cellular quality control mechanisms, which ensure, with variable stringency, that only proteins with the correct structure will proceed through the secretory pathway. Proteins that are misfolded, or not yet folded, are retained in the ER until they reach the native conformation or until their retrotranslocation into the cytosol for degradation. Peculiar characteristic of viruses is their ability to interfere with the cellular machinery to ensure virus production and, moreover, to pass through the body unobserved by the host immune system. This section describes some mechanisms of genetic variation and viral immune evasion that involve the secretory pathway.