| Summary: | Natural transmission of Cucumber necrosis virus (CNV) occurs via zoospores of
the chytrid fungus Olpidium bornovanus. Transmission involves specific adsorption of
virus particles onto the zoospore plasmalemma prior to infestation of cucumber roots by
virus bound zoospores. In order to determine if specific regions of the CNV capsid are
involved in transmission, several naturally occurring CNV transmission mutants were
isolated and characterized. Analysis of the mutants showed that the CNV trimer cavity at
the particle quasi three-fold axis plays an important role in transmission, and, moreover
that the reduction in transmission is at least partially due to the reduced ability of
mutants to bind to zoospores. In vitro virus/zoospore binding studies have shown that
pre-treatment of zoospores with trypsin and sodium periodate each decrease CNV
binding by approximately 80%, whereas no reduction in binding was found when
zoospores were treated with phospholipase C. These studies suggest an important role for
zoospore proteins and/or glycoprotein(s) in virus attachment. In virus overlay assays,
CNV virions bound to specific-sized zoospore proteins, but CNV transmission mutants
showed little or reduced binding. Several sugars were used to study their inhibitory
potential on CNV binding to zoospores in vitro. It was found that a variety of mannose-containing
sugars inhibited CNV binding to zoospores whereas several others did not.
These studies suggest that the putative zoospore receptor may be a mannose-containing
glycoprotein.
Many animal virus particles undergo conformational changes upon binding to
their cellular receptors. CNV is an icosahedral virus and like many other isometric plant
viruses, undergoes expansion in the presence of EDTA at an alkaline pH. In the case of CNV, we have demonstrated that during expansion, the internally located coat protein
RNA binding domain (R) and arm domains translocate to the particle exterior, becoming
protease sensitive. Protease digestion experiments of zoospore-bound virus have revealed
that CNV undergoes conformational change upon binding to zoospores and that the
conformationally altered virion resembles the swollen conformation. In addition, we have
found that a CNV mutant defective in vector transmission is unable to undergo this
conformational change. This is the first time that conformational change in a plant virus
particle has been shown to be essential for vector transmission. === Land and Food Systems, Faculty of === Graduate
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