Year of Publication

2014

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant Pathology

First Advisor

Dr. Peter D. Nagy

Abstract

Positive strand RNA viruses, inculding tombusviruses, are known to utilize cellular membranes to assemble their replicase complexes (VRCs). Two tombusviruses , Tomato bushy stunt virus (TBSV) and Carnation Italian ringspot virus (CIRV), replicate on different organellar membranes, peroxisomes or endoplasmic reticulum (ER) for TBSV and mitochodria outer membranes in case of CIRV. I showed that both TBSV and CIRV replicase proteins could assemble VRCs and replicate viral RNA on purified microsomes (ER) and mitochondria. Different efficiencies of assembly was shown determined by multiple domains on TBSV or CIRV replication proteins.

To study why VRC assembly could occur on an alternative organellar membranes, I focused on the phospholipids, key lipid components in ER or mitochondria membranes. Phospholipids directly interact with viral replicases, however, their specific roles during (+)RNA virus replication are far less understood. I used TBSV as a model (+) RNA virus, and established a cell-free TBSV replication system using artificial membranes prepared from different phospholipids. I showed that phosphatidylethanolamine (PE) is required for full cycle replication of the viral RNA.Moreover, PE is enriched at the sites of TBSV replication in plant and yeast cells, and was up-regulated during TBSV replication. Furthermore, up-regulation of total cellular PE content in yeast due to deletion of CHO2 leads to dramatically stimulated TBSV replication. Overall, I identified PE as the key lipid component of membranes required for TBSV replication, and my data highlighted that PE, an abundant phospholipid in all eukaryotic cells, not only serves as a structural component of membrane bilayers, its interaction with the viral replication proteins also stimulates (+)RNA virus replication. Further experiments indicated both early secretory pathway and endocytic pathway are involved in PE re-distribution to site of replication.

In addition to lipids and subcellular membranes, certain host proteins are also involved in (+) RNA virus replication and VRC assembly. I identified Hop-like stress- inducible protein 1 (Sti1p), which interacts with heat shock protein 70, is required for the inhibition of CIRV replication. My findings indicate that Hop/Sti1 co-chaperone could act as a virus restriction factor in case of mitochondrial CIRV, but not against peroxisomal tombusvirus.

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