The common cold virus uses a unique ‘switch’ during lung infection

Differential cleavage of IRES trans-acting factors (ITAFs) in cells infected by human rhinovirus

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Human rhinovirus (HRV) is a major causative agent of the common cold and has important health implications, especially for individuals suffering from asthma or chronic obstructive pulmonary disease. Understanding HRV RNA replication and how it interacts with host cell functions is crucial for developing antiviral drugs. Like the well-studied poliovirus, HRV is a single-stranded, positive-sense RNA virus in the picornavirus family. Its genomic RNA is used as a template for both translation and negative-strand RNA synthesis. Since it has been shown that genomic RNA templates cannot support both functions simultaneously, a crucial step in the viral replication cycle is the transition in template usage from translation to RNA synthesis. Additionally, picornaviruses have been shown to utilize and/or alter host cell proteins for the benefit of the virus. Our study explored the role of host cell proteins in the HRV intracellular replication cycle, with the goal of determining if viral-mediated cleavage of host proteins is significant, as it has been shown to be important for other picornaviruses.

We found that two host cell proteins involved in picornavirus IRES-mediated translation initiation, PCBP2 and PTB, are proteolytically cleaved by a viral-encoded proteinase during HRV infection of HeLa cells (human cervical carcinoma cells) but not during infection of a human lung fibroblast cell line (WisL cells). Interestingly, both of these proteins are cleaved during poliovirus infection of either HeLa cells or WisL cells, and we have recently provided functional data suggesting that such cleavage events help to mediate a switch in template usage from translation to poliovirus RNA synthesis. The lack of cleavage of PCBP2 or PTB during a fully productive infection of WisL cells suggests that template switching is mediated by a distinct mechanism(s) during HRV infection. These mechanisms might include non-cleavage activities of the virus to reduce the binding of factors required for translation initiation or cleavage of host proteins other than PCPB2 or PTB to down-regulate viral translation. Understanding such mechanisms could have important implications for picornaviruses or other positive-strand RNA viruses (e.g. hepatitis C virus, Dengue virus) since these viruses all require this crucial switch in template RNA usage.

Models for ribosome clearance prior to the initiation of poliovirus and human rhinovirus negative-strand RNA synthesis.

Slide1

Translation of the positive-strand poliovirus genomic RNA (depicted as a black horizontal line with a 3’ poly(A) tract) involves the interaction of host cell proteins known as IRES trans-acting factors or ITAFs (e.g., PCBP2 and PTBs) with stem-loop structures within the internal ribosome entry site (IRES) upstream of the initiator AUG codon.  During poliovirus infection of HeLa cells (human cervical carcinoma cells), these host cell proteins are cleaved by the viral proteinase (3C/3CD), inhibiting viral translation initiation, and allowing the template to be cleared of translating ribosomes prior to its use in viral RNA synthesis.

Slide2

During human rhinovirus infection of WisL cells (human lung fibroblasts), the host cell proteins PCBP2 and PTB are not cleaved. Therefore, there must be an alternative mechanism(s) that promotes ribosome clearance prior to the onset of rhinovirus negative-strand RNA synthesis. As indicated in the figure by the yellow lightning bolt, such mechanisms may involve the cleavage of other, as yet unidentified, ITAFs or initiation factors or non-cleavage functions that alter/reduce ITAF occupancy on human rhinovirus RNA prior to the onset of viral RNA synthesis.

Introducing the authors

Photos for Virology Highlights blog

(L-R) Amanda Chase and Bert Semler, Center for Virus Research and Department of Microbiology and Molecular Genetics, University of California, Irvine, USA

About the research

Differential cleavage of IRES trans-acting factors (ITAFs) in cells infected by human rhinovirus
Virology, Volume 449, 20 January 2014, Pages 35-44
Amanda J. Chase, Bert L. Semler

Read the full article on ScienceDirect.

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