A New Mechanistic Model for Viral Cross Protection and Superinfection Exclusion

Abstract

Plants pre-infected with a mild variant of a virus frequently become protected against more severe variants of the same virus through the cross protection phenomenon first discovered in 1929. Despite its widespread use in managing important plant virus diseases, the mechanism of cross protection remains poorly understood. Recent investigations in our labs, by analyzing the whole-plant dynamics of a turnip crinkle virus (TCV) population, coupled with cell biological interrogation of individual TCV variants, revealed possible novel mechanisms for cross protection and the closely related process of superinfection exclusion (SIE). Our new mechanistic model postulates that, for RNA viruses like TCV, SIE manifests a viral function that denies progeny viruses the chance of re-replicating their genomes in the cells of their "parents," and it collaterally targets highly homologous superinfecting viruses that are indistinguishable from progeny viruses. We further propose that SIE may be evolutionarily selected to maintain an optimal error frequency in progeny genomes. Although primarily based on observations made with TCV, this new model could be broadly applicable to other viruses as it provides a molecular basis for maintaining virus genome fidelity in the face of the error-prone nature of virus replication process.

Keywords: cross protection; p28; protein polymerization; superinfection exclusion; turnip crinkle virus.

FIGURE 1

Genome organization of TCV, and dominant SIE exerted by p28. This figure contains previously published data from our lab (Zhang et al., 2017). (A) Schematic representation of TCV genomic and subgenomic RNAs, and the proteins encoded. (B) Two mutant TCV replicons used in co-infections in (C). Replicon #1 contains deletions within p88 and p8/p9 coding regions, and replacement of p38 by mCherry, thus cannot replicate on its own (top right image, cell boundaries visualized via DAPI staining). Replicon #2 encodes both p28 and p88, and GFP in place of p38, thus replicates in most of treated cells to produce green fluorescence (middle right image). (C) Cells co-infected by replicons #1 and 2. Rescue of replicon #1 in a fraction of cells led to mCherry but not GFP fluorescence, indicating that p28 expressed from the replicating #1 blocks #2, even though the former had to rely on p88 provided by the latter. Note that the constructs were provided in the form of cDNA, thus replicon #2 could be transcribed and translated to provide p88 independent of replication.

FIGURE 2

Model of p28 polymerization at different intracellular concentrations. The small dots represent monomeric p28, whereas the bent sheet represents a section of mitochondrial outer membrane. The left column depicts the situation when p28 concentration is relatively low, causing self-limited polymerization of small lattice on the surface of mitochondrial membrane, which eventually becomes enclosed through membrane curving and invagination. The right column depicts the situation when p28 concentration is high; causing the polymerization to proceed swiftly and the lattice expands quickly, making it impossible to become enclosed by mitochondrial membrane. The exposed lattice, or possibly its multi-layered derivatives, could indefinitely recruit new p28 monomers, blocking them from engaging in replication.

FIGURE 3

Predicted scenario if progeny TCV genomes are permitted to repeat replication in the same cells. Round 1 replication produces progeny genomes that contain one random error per genome on average. Using the middle genome as an example, permitting it to proceed to round 2 would proliferate the round 1 error (red dot) into more progenies, and compound it with additional random errors (yellow dots). Repetition of this process would result in genomes with too many errors that are predicted to be less fit.