Figure 1.

The main mechanism of capsid-targeted viral inactivation (CTVI). (A) Comparison of viral genome encapsidation in a normal virus and a virus containing a capsid–enzyme fusion protein [3]. a. Normal assembly of viruses, whereby nucleic acid is enveloped by capsid proteins to form the nucleocapsid; b. The assembly process of the virus with a fusion protein composed of a capsid protein and a degradative enzyme (e.g., staphylococcal nuclease (SN)) that is calcium ion (Ca²⁺) dependent. The fusion protein is incorporated into the internal virion during viral assembly, where is has direct access to nucleic acid. When the Ca²⁺ concentration reaches the millimolar range, the enzyme is activated and digests the viral RNA/DNA; (B) A schematic representation of the CTVI mechanism. Infection of a cell by a virus stably expressing a fusion protein mainly includes the following steps: a. The virus enters the host cell through the endocytosis pathway; b. the fusion protein is stably expressed; c. using material supplied by the host cell, the fusion protein is incorporated into the viral structure during viral assembly; d. the virus is assembled and modified to form a mature virion within closed vesicles in the cytoplasm, but the nuclease in the virion is inactive due to the intracellular nanomolar Ca²⁺ concentration; e. the virus is released into the extracellular environment; f. the nuclease incorporated into the progeny virion is active in the extracellular millimolar Ca²⁺ concentration, where it can degrade the viral nucleic acids.