Picornaviridae

Poliovirus, in the family picornaviridae. © Dennis Kunkel Microscopy, Inc.

Baltimore Classification

Higher order taxa

Virus; ssRNA positive-strand viruses, no DNA stage; Picornaviridae

Genera

Enterovirus, Rhinovirus, Hepatovirus, Apthovirus, Cardiovirus, Parechovirus, Erbovirus, Kobuvirus, and Teschovirus

Description and Significance

The name "picornavirus" means " little RNA virus" (pico means "very small" in Greek; pico + RNA virus). Picornaviruses are among the oldest viruses, dating back to 1400 B.C., when a temple record tells of poliovirus infections in ancient Egypt. Picornaviruses are also among the most diverse viruses, with over 200 serotypes causing infections such as Polio, Hepatitis A, and the common cold. Foot-and-Mouth Disease Virus (genus: Apthovirus), which causes infections in livestock, was one of the first viruses to be recognized; it was discovered by Loeffler and Frosch in 1898. (sources: Flynn, Bedard)

Genome Structure

The picornavirus genome consists of a single molecule of linear, postitive(+)-sense, single-strand RNA. It is non-segmented. The complete genome is 7000-8500 nucleotides long. At the 5'-terminus of the genome has a long untranslated region 600-1200 bases in length, which is important in translation, virulence, and possibly encapsidation. There is a shorter untranslated region (50-100 bases in length) on the 3'-terminus, which is important in (-)strand-synthesis. The 5'-terminus untranslated region also has a "clover leaf" secondary structure known as the Internal Ribosome Entry Site (IRES), which distinguishes picornaviruses from other RNA viruses; this structure is important in translation and replication. The 5'-terminus is modified by a covalently-attached VPg protein (which takes the place of a cap), while the 3'-terminus is modified by polyadenylation. (sources: ICTVdB, Bedard, Flynn)

Virion Structure of a Picornavirus

Picornavirus virions consist of a non-enveloped, icosahedrally symmetric capsid. The capsid consists of 12 capsomers and has a diameter of 27-30nm, which makes it one of the smallest of all viruses (thus the name "picornavirus"). The genome is tightly packed into the capsid. The capsid has four unique proteins: VP1, 2, 3, and 4. (sources: ICTVdB, Flynn)

Reproductive Cycle of a Picornavirus in a Host Cell

Using different cellular receptors (depending on the picornavirus), a picornavirus virion attaches to a host cell. Uncoating occurs, and the virus' RNA is released into the cytoplasm of the host cell through a membrane channel. Virus replication occurs entirely in the cytoplasm. The host cell's transcription processes are shut off to a degree that varies with different picornaviruses, while the IRES helps to make sure the virus' transcription is left untouched. Replication occurs. RNA is packaged into preformed capsids. Release of the virus occurs when cell lysis occurs (with the exception of Hepatits A, which is non-lytic and thus creates a more persistent infection). (source: Bedard)

Viral Ecology & Pathology

The mode of transmission, ecology, and pathology of picornaviruses vary greatly between the different genera. Interestingly, the genetics of virulence phenotypes of picornaviruses is poorly understood. According to Tracy et al., "Picornaviruses do not have virulence genes per se, but the design of the capsid and how it interacts with the virus receptor expressed on the host cell surface, specific sequences within the nontranslated regions of the viral genome, as well as coding sequences that result in different protein sequences may all have a part in determining the virulence phenotype."

References

Bedard, Kristin M. and Bert L. Semler. "Regulation of picornavirus gene expression." Microbes and Infection 6.7 (2004): 702-713.

Flynn, Emily. Picornaviridae.

ICTVdB - The Universal Virus Database, version 4. http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/

Tracy et al. "Evolution of virulence in picornaviruses." Curr Top Microbiol Immunol 299 (2006): 193-209.