Date Available

6-11-2021

Year of Publication

2021

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Medicine

Department/School/Program

Molecular and Cellular Biochemistry

Advisor

Dr. Rebecca Dutch

Abstract

Paramyxoviruses, pneumoviruses, and other non-segmented negative sense (NNS) RNA viruses have historically been of public health concern. Although their genomes are typically small (up to 19kbs) they are able to inflict large-scale detrimental pathologies on host cells. Human metapneumovirus (HMPV) is a widespread pathogen and is a NNS RNA virus. HMPV results respiratory tract infections and is particularly dangerous for preterm infants, the elderly, and immunocompromised individuals. Other viruses within the NNS RNA virus order include the deadly Ebola, Hendra, and Nipah viruses (EBOV, HeV, and NiV), as well as the re-emerging measles virus (MeV). Despite their public impact, there are currently very limited available FDA-approved therapeutics and antivirals against NNS RNA viruses. During the infectious cycle, viral surface glycoproteins play critical roles in establishing infection.

For most NNS RNA viruses, the attachment protein is important for the tethering of a viral membrane to host cells, while the fusion protein is responsible for the membrane merger of the virus and host. The fusion protein of paramyxo-and pneumovirus proteins are class I proteins that are folded into trimers, must be proteolytically cleaved to be functional, and are held in a metastable prefusion conformation until the signal for fusion occurs. Upon being signaled, the fusion protein undergoes dramatic essentially irreversible conformational changes for membrane mixing. Because of its important role in starting infection, F has garnered interest as a potentially powerful target against infection. For paramyxoviruses, the ectodomain regions of F have been well-studied; however, the hydrophobic nature of the transmembrane domain (TMD) of the protein has resulted in difficulties in crystallization. To address this, several biochemical assays have been utilized to address the function of the TMDs of paramyxo-and pneumovirus fusion proteins. Although initially thought to be solely a membrane anchor, the transmembrane domains of several viruses have been shown to be important for the functionality of fusion proteins. For some paramyxoviruses, replacement of the proteinaceous TMD resulted in the premature triggering. Further studies showed that the TMDs of paramyxoviruses and several other viral F proteins exist in isolation as trimers, and these trimeric associations in turn drive trimeric associations of the full protein. Studies of the HeV F TMD in isolation identified a leucine/isoleucine (L/I) zipper as an important motif for TMD-TMD trimerization. Mutations to this L/I zipper motif in the context of the full protein resulted in reduced surface expression, and a loss of functionality. The L/I zipper was found to be present in 140 paramyxo- and pneumovirus fusion protein TMDs. This work examines whether wh iimporether the importance of the L/I zipper in the context of another paramyxvovirus. We used the model system, PIV5 F to dissect the role of the TMD L/I zipper in expression and fusogenic activity. We found that the (L/I) zipper plays important roles in functionality of the PIV5 F protein, but not surface expression of the protein.

Following membrane merging, a series of events occur that facilitate the release of viral contents into the host cell. The NNS RNA carried by the virus into the cell is used as a template for viral replication and transcription; two important steps in generation of viral progeny. In the life cycle of NNS viruses, viral proteins assume multi-functional roles to optimize their replication and spread. One of the key players during the course of infection is the matrix protein (M). The matrix protein has been identified as a master regulator of viral infection with most studies focusing on its roles in late-stage infection, during assembly and budding of viral progeny. The matrix proteins of many enveloped viruses have been shown to associate in high order oligomers to form a grid- like array underneath the plasma membrane, where they can induce membrane curvature to allow for the budding of viral particles. Not surprisingly, the absence of M in some NNS RNA viruses results in a significant viral titer decrease. Interestingly, some recent studies show that the matrix protein has other critical roles in viral infection such as immune modulation and host cell translation antagonism. One of these newly uncovered roles for viral matrix proteins involves the regulation of viral RNA synthesis. Studies with EBOV and MeV demonstrate that the matrix protein is involved in early infection events, as inhibits viral replication. To study the roles of the HMPV M protein in early infection, we performed a spatiotemporal analysis of M in HMPV-infected cells. We noted the presence of HMPV M within the nucleus during early infection. Our knockdown studies of HMPV M indicate that HMPV M is a positive regulator of viral replication and transcription, as in its absence, the rates of mRNA and viral genomic RNA synthesis are dramatically reduced. Additionally, within the NNS RNA virus order, HMPV M is the only matrix protein found to bind calcium. We created alanine mutants to the calcium coordinating residues of HMPV M and found that these residues were important in properly folding the protein. Together, these findings contribute to our understanding of the mechanisms of NNS RNA viral infection.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.248

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