Molecular biology study of satellite panicum mosaic virus capsid protein

摘要

Satellite panicum mosaic virus (SPMV) depends on its helper Panicum mosaic virus(PMV) for replication and movement in host plants. The positive-sense single-strandedgenomic RNA of SPMV encodes a 17-kDa capsid protein (CP) to form 16-nm virions.Previous studies showed that SPMV CP has multiple functions during infectionincluding encapsidation, symptom exacerbation, inhibiting the accumulation of SPMVDIs, and facilitating systemic movement.This dissertation confirms and extends the results of our previous reports with newbiological and biochemical evidence. For example, the dosage effect of SPMV CP onsymptom severity supports its function as a pathogenicity factor. Biological assays alsodemonstrate compensatory effects of SPMV CP on virus mutants defective in systemicmovement. In addition, it is shown for the first time that SPMV CP is involved in cellto-cell movement of SPMV RNA and associated with the cell wall and membranes, asignature property of plant virus movement proteins. However, SPMV CP in the cytosolexists exclusively as virions and is dispensable for symptom exacerbation. SPMV CP contains a distinctive N-terminal arginine-rich motif (N-ARM), which isrequired for the in vitro binding of SPMV and PMV genomic RNAs by SPMV CP.Mutations of this region impair all known functions of SPMV CP. Interestingly,manipulation of the C-terminus of SPMV CP resulted in the same phenotypes asalterations in the N-ARM except that this does not affect the RNA binding activity ofSPMV CP. Biological experiments demonstrate that virions are not required for theproperties of SPMV CP to facilitate local and systemic movement and inhibit theaccumulation of SPMV DIs, suggesting that SPMV CP and RNA form alternativecomplexes for these purposes. This dissertation study reveals the nucleolar localizationof SPMV CP and its interaction with PMV CP in the form of virions.The identification of distinct functional domains of SPMV CP and its complexsubcellular localization profile resulted in the proposal of a tentative model on how thefunctions of SPMV CP are coordinated for a robust infection. This dissertation providesa foundation for further understanding of the complex interactions among host plants,helper viruses, and satellites.