Coronaviruses. Virion. Genome. Genes and proteins. Viruses and hosts. Diseases. Distinctive characteristics

Transcription

1 Coronaviruses Virion Genome Genes and proteins Viruses and hosts Diseases Distinctive characteristics

2 Virion Spherical enveloped particles studded with clubbed spikes Diameter nm Coiled helical nucleocapsid and/or nucleocapsid shell

3 Genome Linear single-stranded RNA, positive sense kb 5 terminal cap, 3 poly(a) tail

4 Genes and proteins Six to nine genes code for more than 20 proteins Replicase genes are translated directly from genome RNA Other genes are translated from multiple, 3 co-terminal, subgenomic mrnas

5 Viruses and hosts Subfamily Coronaviridae: from Latin (crown) Three genera based on genome homologies Humans: human coronaviruses 22 E, OC43, NL63, HKU-1, SARS coronavirus Animals: murine hepatitis virus, transmissible gastroenteritis virus, avian infections bronchitis virus

6 Diseases Coronaviruses cause respiratory illnesses in humans and important veterinary diseases Up to 30% of common colds in humans and rarely cause gastroenteritis and encephalitis Feline infectious peritonitis Infectious bronchitis in animals Murine hepatitis

7 Diseases A newly emerged coronavirus caused a worldwide epidemic of severe acute respiratory syndrome (SARS) Caused fever and progressive pneumonia which led to hypoxemia, and death 8000 illnesses and 800 deaths Mortality rate of 44% in people 65+ Transmitted by direct contact, aerosol droplets, contaminated stool

8 Diseases SARS coronavirus may have originated from related bat coronavirus Viruses similar to SARS found in many types of animals sold in open-air markets (China) A huge number of coronaviruses have been found in bats a possible animal reservoir

9 Diseases How did a bat coronavirus mutate and enter humans to become SARS coronavirus? Possibilities: Virus passed from bats to animals, who served as amplifying hosts allowing viral mutation before spread to humans Nonpathogenic virus passed from bat to humans, then to animals where it mutated to become pathogenic

10 Genome Coronaviruses have large, singlestranded, positive-sense RNA genomes First gene translated as a polyprotein and subsequently cleaved

11 Viruses and hosts Coronaviruses fall into three groups based on genome sequences Family Coronaviridae has three genera Alpha-, beta- and gammacoronaviruses

12 Viruses and hosts

13 Virion Coronaviruses have enveloped virions containing helical nucleocapsids Nucleocapsids formed from N protein bound to viral RNA in a helical fashion Some coronaviruses may have a spherical core shell

14 Virion Coronavirus virions contain multiple envelope proteins Spike protein (S) involved in entry, spread and host range Hemagglutinin-esterase protein (HE) may play a role in binding, entry and release Membrane protein (M) Small envelope protein (E) may facilitate budding of virus into ER/Golgi

15 Genes and proteins Coronavirus spike proteins bind to a variety of cellular receptors Binds to aminopeptidase-n (a family of zincbinding metalloproteinases) Murine hepatitis virus binds to CEACAM1 SARS binds to ACE2, L-SIGN (a lectin) Viruses that have HE can bind to sialic acid

16 Distinctive characteristics The virus envelope fuses with the plasma membrane or an endosomal membrane S1 mediates attachment S2 (a class I fusion protein) facilitates fusion S proteins can cause syncytia formation

17 Distinctive characteristics The replicase gene is translated from genome RNA into a polyprotein that is processed by viral proteinases Large gene 1 is comprised of ORF1a and ORF1b Ribosomes translating ORF1a pause at a pseudoknot and frameshift allowing translation of ORF1b

18 Distinctive characteristics Viral proteinases include: papain-like cystein proteinase and a chymotrypsin-like cystein proteinase (resembling picornavirus 3C proteinase) RNA polymerase, RNA helicase and RNAmodifying enzymes are encoded by the replicase gene

19 Genes and proteins

20 Genes and proteins

21 Distinctive characteristics Replication complexes are associated with cytoplasmic membranes Replication complexes are the site of viral RNA synthesis Membrane structures are virally-induced Encapsidation of RNA also occurs at these sites

22 Distinctive characteristics A. An early double membrane vesicle showing a connection (arrow) to a reticular membrane B. A cluster of doublemembrane vesicles, showing a connection between the outer membranes and reticular membrane structures (arrow)

23 Distinctive characteristics Genome replication proceeds via a fulllength negative-strand intermediate Negative-strand RNAs account for 1-2% of total viral RNA Most negative-strands are associated with one or more growing positive-strands in a replicative intermediate

24 Distinctive characteristics Transcription produces a nested set of subgenomic mrnas The expression of all genes downstream of the replicase gene occurs from a series of subgenomic mrnas

25 Distinctive characteristics The subgenomic mrnas all have identical leader sequences at the 5 end, poly(a) tails, and represent a nested set of mrnas Each mrna is translated to produce one or more proteins

26 Distinctive characteristics Subgenomic mrnas are transcribed from subgenomic negative-sense RNA templates made by discontinuous transcription Polymerase begins at 3 end and stops at the end of a transcription-regulating sequence (TRS) Each of the subgenomic (-) strands are used to make a (+) mrna

27 Distinctive characteristics Discontinuous transcription of genome mrna

28 Distinctive characteristics The discontinuous model can explain recombination between viral genomes RNA pol can switch between two different positive-strand RNAs if they are both in the same cell

29 Distinctive characteristics Assembly of virions takes place at intracellular membrane structures Endoplasmic reticulum- Golgi intermediate compartment (ERGIC)

30 Distinctive characteristics Adaptability and evolution of coronaviruses Subtle changes can alter tissue tropism, host range and pathogenicity Changes occur by: High theoretical error rate of RNA polymerase RNA-RNA recombination

31 Fundamental concepts Coronaviruses are responsible for common colds, veterinary diseases, and an outbreak of SARS Coronaviruses have the largest known RNA genomes Coronaviruses have helical nucleocapsids, while some have an additional spherical capsid The replicase genes are translated to produce two overlapping polyproteins, which are cleaved All structural proteins are made from a 3 coterminal nested set of mrnas

32 Fundamental concepts The subgenomic mrnas are copies of subgenomic RNAs transcribed by a discontiuous transcription mechanism Coronaviruses induce intracellular membrane proliferation where viral RNA replication takes place Coronavirus virions are assembled within the ERGIC and secreted via vesicles that fuse with the cell membrane

33 Key terms Bronchitis Class I fusion proteins Class II fusion proteins Coiled-coil Discontinuous transcription Encephalitis ERGIC Gastroenteritis Hepatitis Hypoxemia Lectin Metalloproteinase Nested set Papain-like cysteine proteinases Peritonitis Picornavirus 3C proteinase Polyprotein Pseudoknot Replication complexes Replicative intermediate Reservoir Reverse genetics Sialic acid Subgenomic mrna Syncytium Tropism