Capsid-Targeted Viral Inactivation: A Novel Tactic for Inhibiting Replication in Viral Infections

Abstract

Capsid-targeted viral inactivation (CTVI), a conceptually powerful new antiviral strategy, is attracting increasing attention from researchers. Specifically, this strategy is based on fusion between the capsid protein of a virus and a crucial effector molecule, such as a nuclease (e.g., staphylococcal nuclease, Barrase, RNase HI), lipase, protease, or single-chain antibody (scAb). In general, capsid proteins have a major role in viral integration and assembly, and the effector molecule used in CTVI functions to degrade viral DNA/RNA or interfere with proper folding of viral key proteins, thereby affecting the infectivity of progeny viruses. Interestingly, such a capsid-enzyme fusion protein is incorporated into virions during packaging. CTVI is more efficient compared to other antiviral methods, and this approach is promising for antiviral prophylaxis and therapy. This review summarizes the mechanism and utility of CTVI and provides some successful applications of this strategy, with the ultimate goal of widely implementing CTVI in antiviral research.

Keywords: Capsid-targeted viral inactivation; antiviral strategy; core protein; degradative enzyme; fusion proteins.

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.

Figure 2

Gag protein components. The structural protein Gag is mainly processed into four proteins, p17 matrix (MA), p24 capsid (CA), p9 nucleocapsid (NC), and P6, by a virus-encoded protease.