Compare the life cycles of an orthomyxovirus and a coronavirus from entry to release PDF

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Compare the life cycles of an orthomyxovirus and a coronavirus from entry to release: Both orthomyxoviruses and coronaviruses are capable of causing significant disease in humans, however, the mechanisms by which they carry out their intracellular lifecycles have many differences as well as similarities. Here, the influenza and SARS-CoV will be used as examples of how these groups of viruses compare from host cell entry to release. Influenza is a member of the orthomyxovirus species of viruses and has a -ssRNA genome. This is 13kb long and is made up of 8 segments that are able to undergo genetic reassortment that causes antigenic shift responsible for seasonal flu, outbreaks, and pandemics. Attachment occurs through the HA protein that binds sialic acid receptors of target respiratory tract cells. This triggers receptor mediated endocytosis that results in the host cell attacking the incoming cargo through acidification. However, once the pH drops the viral core becomes destabilise through protons that enter via the M2 proton channel and partially unfolds the HA. This allows cleavage into HA1 and HA2 by host proteases and exposes the class I fusion peptide. This inserts into the membrane to fuse with the endosomal membrane to translocate the viral contents into the cytoplasm. Influenza is one of the few groups of RNA viruses that undergo replication within the nucleus. Its genome is bound by proteins in a viral ribonucleoprotein complex for packaging and protection. These segmented vRNP act as a template for vRNA and cRNA. vRNA is produced through the actions of the three units of viral RNA polymerase. However, because the influenza genome is uncapped, the vRNA requires a mechanism to gain a cap in order to be translated. This occurs through cap stealing mediated by the polymerase. Firstly, the PB1 subunit binds the 5’and 3’ ends of the newly produced vRNA followed by the PB2 subunit binding 5’ end of host cell mRNA. Then endonuclease activity leads to the cleavage of these host mRNAs 10-13 bases from the end at an adenosine. This cap is then base paired to the vRNA and the chain elongated by the activity of PB1. This terminates once it reaches a stretch of uridines that causes reiterative transcription and generates a poly-A-tail. The cRNA acts as a perfect anti-sense template for the viral genome and is neither capped nor polyadenylated. It can either be used for the synthesis of more vRNA or packaged into progeny. Splicing can occur to allow the viral genome to encode multiple different proteins. Segment 7 is able to encode both the M1 matrix protein and M2 ion channel. Translation for M1 is through normal exportation of the mRNA into the cytoplasm and use of host machinery. Splicing removes a segment and repositions the ORF of M2 next to the start codon through the presence of donor and acceptor sites which mimics removal of an intron to allow the production of the M2 protein. Leaky translation and stop-start cistrons are additional mechanisms encoded within the genome. Leaky scanning is a mechanism by which the ribosome is able to bypass the initial AUG start codon and begin translation further downstream at another start codon that lies in a different frame to result in the production of an alternate set of proteins. The stop-start cistrons occur at sequences like UAAUG where the ribosome can either read an UAA stop codon or an AUG start which leads to either termination or reinitiation of synthesis. Viral assembly occurs with the influenza as the M1 protein is translocated to the nucleus where it interacts with the vRNP complexes and transports them via microfilaments to the apical membrane. It is at this membrane where the HA, NA, and M2 proteins are inserted. M1 then introduces bending in the membrane and the virus begins to bud off. The virus is released by the NA cleaving the interaction between HA and the sialic acid receptor and the virus becomes fully matured.

SARS-CoV is an example of a coronavirus and with genome of 30kb is one of the largest known RNA virus and has +ssRNA genome. It is the causative agent of severe acute respiratory syndrome and has been implemented in many epidemics which include a notable outbreak in China during 2003. The intracellular lifecycle of this virus begins with the binding of viral S proteins to the ACE-2 enzyme on the membrane of host cells that triggers receptor mediated endocytosis just like with influenza. The subsequent acidification of the endosome results in the release of the genome through membrane fusion involving this S protein. Just like HA it is also a trimeric class I fusion protein but contains domains S1 and S2 that does not require cleavage. Unlike influenza the genome of this virus contains a 5’ guanine cap and a 3’ poly-A-tail and is made up of OFR 1a and ORF 1b. These ORFs act as polyproteins and encode various enzymes involved in genomic replication like RdRp, helicase, and an exonuclease. Downstream of these ORFs lay the structural proteins that are the spike, nucleocapsid, membrane associated protein, and envelope associated protein. SARS-CoV is able to replicate within in the cytoplasm with host machinery whereas influenza replicates within the nucleus. It has a complex way of producing proteins that mediate replication of the genome and form the viral structure. Firstly, frameshifts are an essential mechanism used for the expression of ORF 1ab. This is through the inclusion of a slippery sequence (UUUAAAC) followed by a downstream pseudoknot that occurs in the region where the ORFs overlap. This 3D structure needs to be resolved if the ribosome is to carry on translating. This means that either the ribosome will terminate translation here and only produce ORF1a, or it will resolve the structure and frameshift back a single base to produce ORF1ab. Polyprotein processing is not utilised by influenza but allows SARS-CoV proteases to cleave the subsequent proteins into functional enzymes needed for other processes. The newly produced replicase complex contains RNA polymerase activity that can transcribe the full length –ssRNA that acts as a template for the transcription of the full length +ssRNA that is packaged into progeny. The production of mRNA uses leader-body fusion to ensure they all have the same 60-70kb 5’ cap sequence vital for ribosome attachment for protein production. This is followed by a TRS found multiple times throughout the genome. Subgenomic mRNA is produced from the +ssRNA as the complex binds and transcribes until encounters a TRS. Either the complex continues synthesis past this region until another TRS or it dissociates and rebinds at the 5’ end to transcribe the leader sequence. This -ssRNA acts as the template for mRNA that is used to produce structural proteins. The efficiency of the TRS determines the relative composition of the protein profile. The N protein and the full length genome of SARS-CoV assemble in the cytoplasm and interact with the M protein that acts as a chaperone. This guides these components to bud into the endoplasmic reticulum to form viruses to the plasma membrane via the Golgi apparatus where they are packaged into vesicles and exocytose to release mature particles....