Lesson 7 Viruses, Viroids, and Prions February 19, 2015

General Characteristics of Viruses Obligatory intracellular parasites—require living host cells in order to multiply Some features of a virus are: Contain DNA or RNA Do not contain both Contains an encapsulating protein coat Some viruses have spikes

General Characteristics of Viruses Uses host cell machinery to replicate Why is this problematic for humans???? Have a varied host range (cells that it infects) determined by specific host attachment sites and cellular factors Viruses that infect bacteria are called bacteriophages

Chlamydia elementary body Figure 13.1 Virus sizes. 225 nm Human red blood cell 10,000 nm in diameter Bacteriophage T4 Rabies virus 170 × 70 nm Bacteriophage M13 800 × 10 nm Adenovirus 90 nm Rhinovirus 30 nm Chlamydia elementary body 300 nm Tobacco mosaic virus 250 × 18 nm Bacteriophages f2, MS2 24 nm Viroid 300×10 nm Prion 200 × 20 nm Poliovirus 30 nm Vaccinia virus 300 × 200 × 100 nm Ebola virus 970 nm E. coli (a bacterium) 3000 × 1000 nm Plasma membrane of red blood cell 10 nm thick

Virion Structure Virion—complete, fully developed viral particle Nucleic Acid DNA or RNA (double stranded or single stranded) Positive-strand RNA(uses host’s RNA polymerase) vs Negative stranded RNA (encodes own RNA polymerase) Capsid—protein coat that protects nucleic acid Capsomeres—small protein subunits of the capsid Arrangement of capsomeres is virus-specific

Nucleic Acid Capsomere Capsid A polyhedral virus Mastadenovirus Figure 13.2 Morphology of a nonenveloped polyhedral virus. Nucleic Acid Capsomere Capsid A polyhedral virus Mastadenovirus

Virion Structure Envelope—surrounds the capsid Comprised of lipids, proteins, and carbohydrates Envelopes are comprised of the host cell’s plasma membrane (extrusion) Some viruses are non-enveloped Capsid protects genetic material from nucleases Spikes—carbohydrate-protein complexes that protrude from the envelope. Viruses can escape the immune system by mutating the surface proteins!!!! Influenza (flu-vaccine not 100% effective due to mutations)

Taxonomy of Viruses Family names end in -viridae Genus names end in -virus Family Herpesviridae, genus Simplexvirus Viral species: a group of viruses sharing the same genetic information and ecological niche (host) Common names are used for species Subspecies are designated by a number Human Immunodeficiency Virus subspecies I (HIV-1)

Growing Viruses Viruses MUST be grown in living cells Why is this? Bacteriophages form plaques on lawn of bacteria Plaques—clearing of bacteria. Concentration of viral suspension measured by plaque-forming units (PFUs) Animal viruses may be grown in living animals or in embryonated eggs or in cell cultures Continuous cell lines (HeLa cells) Influenza for flu vaccine are grown in embryonic eggs (primary cell lines)

Figure 13.6 Viral plaques formed by bacteriophages.

Chlorioallantoic membrane inoculation Air sac Figure 13.7 Inoculation of an embryonated egg. Amniotic cavity Chlorioallantoic membrane Shell Chlorioallantoic membrane inoculation Air sac Amniotic inoculation Yolk sac Allantoic inoculation Shell membrane Yolk sac inoculation Albumin Allantoic cavity

Virus Identification Cytopathic Effects—visual effects seen in viral infected cell Cell rounding Cell lysis Cell clumping (multiple nuclei in one cell) Serological Tests Detect antibodies against viruses in a patient (ELISA) Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot Nucleic Acids RFLPs PCR

Viral Multiplication In order for a virus to multiply it must invade a host cell and use the host’s metabolic machinery Host cell can be ruptured to release virus (Lytic cycle) or survive to continue producing viruses indefinitely (Lysogenic Cycle) Unlike bacteria, viruses have a one-step growth curve

Figure 13.10 A viral one-step growth curve. Acute infection Virions released from host cell Number of virions Eclipse period Time (days)

Results of Multiplication of Bacteriophages Viral infection can be categorized into two different processes Lytic cycle Phage causes lysis and death of host cell Lysogenic cycle Prophage DNA incorporated in host DNA Phage conversion—host cell exhibiting new properties Some bacteria are only pathogenic when they contain prophage DNA Shiga toxin in E. coli and exotoxin of S. aureus that causes Toxic Shock Syndrome

The Lytic Cycle Attachment: phage attaches by tail fibers to host cell Penetration: phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell Biosynthesis: production of phage DNA and proteins Maturation: assembly of phage particles Release: phage lysozyme breaks cell wall

Figure 13.11 The lytic cycle of a T-even bacteriophage. Bacterial cell wall Bacterial chromosome Capsid DNA Capsid (head) Sheath Attachment: Phage attaches to host cell. 1 Tail fiber Tail Baseplate Pin Cell wall Plasma membrane Penetration: Phage penetrates host cell and injects its DNA. 2 Sheath contracted Tail core Biosynthesis: Phage DNA directs synthesis of viral components by the host cell. 3 Tail DNA Maturation: Viral components are assembled into virions. 4 Capsid Tail fibers Release: Host cell lyses, and new virions are released. 5

Lytic cycle Lysogenic cycle Figure 13.12 The lysogenic cycle of bacteriophage λ in E. coli. 1 Phage attaches to host cell and injects DNA. 5 Occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle. Phage DNA (double-stranded) Bacterial chromosome Many cell divisions Lytic cycle Lysogenic cycle 4A Cell lyses, releasing phage virions. 2 Phage DNA circularizes and enters lytic cycle or lysogenic cycle. 4B Lysogenic bacterium reproduces normally. Prophage OR 3A New phage DNA and proteins are synthesized and assembled into virions. 3B Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage.

Enveloped vs. Non-Enveloped Virus Figure 13.15 Replication of a DNA-Containing Animal Virus. 1 ATTACHMENT Virion attaches to host cell. A papovavirus is a typical DNA-containing virus that attacks animal cells. 7 RELEASE Virions are released. Papovavirus DNA 2 ENTRY and UNCOATING Virion enters cell, and its DNA is uncoated. Host cell Capsid 6 MATURATION Virions mature. Nucleus Cytoplasm Capsid proteins Viral DNA Capsid proteins 4 BIOSYNTHESIS Viral DNA is replicated, and some viral proteins are made. mRNA 5 Late translation; capsid proteins are synthesized. 3 A portion of viral DNA is transcribed, producing mRNA that encodes “early” viral proteins. Enveloped vs. Non-Enveloped Virus

Differences between Animal Virus and Bacteriophage Infection

Latent vs. Persistent Infections Latent viral infections are infections where the viral DNA is integrated into the host May not produce disease for a long period of time Reactivated via immunosupression or exogenous stimulus Herpesviruses Reactivated by fever or sunburn (fever blister) Varicellovirus (chickenpox) Acquired in childhood but remains dormant in nerve cells Changes in T-cell response activate the virus (Shingles)

Latent vs. Persistent Infections Persistent viral infections occurs gradually over a long period Also called chronic viral infections Viruses gradually are produced over a long period of time Usually fatal HIV and measles virus are examples of persistent infections

Latent and Persistent Viral Infections

Prions Proteinaceous infectious particle Initially discovered in sheep and called scrapie Infectivity in sheep’s brain reduced by proteases and not radiation Inherited and transmissible by ingestion, transplanted nerve tissue, and surgical instruments Spongiform encephalopathies: Creutzfeldt-Jakob disease, kuru, mad cow disease Caused by the conversion of a normal host glycoprotein PrPC into an infectious form PrPSC

Figure 13.22 How a protein can be infectious. PrPc PrPSc 1 PrPc produced by cells is secreted to the cell surface. 2 PrPSc may be acquired or produced by an altered PrPc gene. 3 PrPSc reacts with PrPc on the cell surface. 4 PrPSc converts the PrPc to PrPSc. 5 The new PrPSc converts more PrPc. 6 The new PrPSc is taken in, possibly by receptor-mediated endocytosis. Lysosome 7 8 PrPSc continues to accumulate as the endosome contents are transferred to lysosomes. The result is cell death. PrPSc accumulates in endosomes. Endosome

Creutzfeldt-Jakob disease

Plant Viruses and Viroids Plant cells are normally protected via their cell walls therefore viruses enter through wounds or via insects Insects can serve as reservoirs for some plant viruses Causes many diseases to economically important crops

Viroids Short pieces of infectious RNA Lacks a protein coat 300-400 nucleotides long Do not encode proteins Possibly derived from introns Lacks a protein coat Internally paired (forms stem-loops) that protect it from degradation Example: Potato spindle tuber disease

Figure 13.23 Linear and circular potato spindle tuber viroid (PSTV).