Nidoviruses

Nidoviruses

  • Replication

Nidoviruses are single stranded positive RNA viruses (from now on named as +RNA). They replicate in the cytoplasm of the host after infection. Modified cell membranes are coupled to the replication complexes of these viruses.

Nidoviruses transform into a unique reticulovesicular network of the endoplasmatic reticulum (ER) of the host. It brings together various double-membrane vesicles (DMVs). In case of coronaviruses: convoluted membranes (CM). The interior of DMV contains double-stranded RNA and is therefore probably the site of RNA synthesis if the virus. [1]

Reasearch has shown that the ds RNA is protected by a membrane.

These viruses recognize the infrastructure of their host cell’s membrane to correlate its replication cycle and use it to protect their own replicating RNA from defence mechanisms. [2]

 

nidovirus replication

 

Figure 1: SARS-CoV is a coronavirus – a family of the nidoviruses. It has many similarities with MHV. [3]

The virus enters the host cell after binding to the ACE2 receptor on the membrane. The RNA genome is released into the cytoplasm where it is replicated. Polyproteins 1a and 1ab (pp) are produced from the ORF1a/b, which result in loose subunits: the nsps. The replicated mRNA can be translated to form the S-proteins (located on the outside of the viral envelope. They bind to the receptor) into the ER and Golgi apparatus. When enough RNA is replicated and proteins are produced, new virions are assembled and packaged to move outside the host, without lysing it.

pp1ab

Figure 2: coronavirus replicase polyprotein (pp1ab)

The replicase cleavage products (nsp1-16) are indicated with numbers. Furthermore, the conserved domains are highlighted: the blue ones are conserved in nidoviruses, whereas the grey ones are  conserved in coronaviruses. RFS stands for the ribosomal frame shift of ORF1a and ORF1b. [2]

 

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