Question

1.Consider the proteinaceous spikes (peplomers) on enveloped viruses such as the Herpesvirus.

a) What is the genetic origin of these sipke proteins?

b) Describe the machanism that results in these peplomers occuring on the virion envelope

Answer 1

1The genetic origin of the spike protein is the evolution and population dynamics of avian coronaviruses (AvCoVs) remain underexplored. In the present study, in-depth phylogenetic and Bayesian phylogeographic studies were conducted to investigate the evolutionary dynamics of AvCoVs detected in wild and synanthropic birds.

The spike protein (S protein) is a large type I transmembrane protein ranging from 1,160 amino acids for avian infectious bronchitis virus (IBV) and up to 1,400 amino acids for feline coronavirus (FCoV) (Figure 1). In addition, this protein is highly glycosylated as it contains 21 to 35 N-glycosylation sites. Spike proteins assemble into trimers on the virion surface to form the distinctive "corona", or crown-like appearance. The ectodomain of all CoV spike proteins share the same organization in two domains: a N-terminal domain named S1 that is responsible for receptor binding and a C-terminal S2 domain responsible for fusion (Figure 2). CoV diversity is reflected in the variable spike proteins (S proteins), which have evolved into forms differing in their receptor interactions and their response to various environmental triggers of virus-cell membrane fusion.

A notable distinction between the spike proteins of different coronaviruses is whether it is cleaved or not during assembly and exocytosis of virions. With some exceptions, in most alphacoronaviruses and the betacoronavirus SARS-CoV, the virions harbor a spike protein that is uncleaved, whereas in some beta- and all gammacoronaviruses the protein is found cleaved between the S1 and S2 domains, typically by furin, a Golgi-resident host protease. Interestingly, within the betacoronavirus mouse hepatitis virus (MHV) species, different strains, such as MHV-2 and MHV-A59 display different cleavage requirements. This has important consequences on their fusogenicity.

2.The CoVs are widely distributed in nature and their zoonotic transmissions into human populations can cause epidemic disease. After entering into respiratory or gastrointestinal tracts, these viruses establish themselves by entering and infecting lumenal macrophages and epithelial cells. The cell entry programs for these viruses are orchestrated by the viral spike (S) proteins that bind cellular receptors and also mediate virus-cell membrane fusions. Take SARS-CoV for example. The spike protein (S protein) of SARS-CoV has pivotal roles in viral infection and pathogenesis. S1 recognizes and binds to host receptors, and subsequent conformational changes in S2 facilitate fusion between the viral envelope and the host cell membrane.

Models depicting the S-mediated membrane fusion event have extended from knowledge of S protein structures and functions. In part, these models are deemed reasonable because the postfusion 6-HB conformations in SARS and MHV S proteins are so strikingly similar to postfusion forms of influenza HA2, paramyxovirus F2, Ebolavirus GP2 and HIV gp41. In analogy to these more widely-studied and well-understood viral fusion proteins, the CoV S-mediated membrane fusion process is generally viewed as schematized

Schematic of CoV spike protein mediated membrane fusion. The illustrations represent several steps of S protein conformational changes that may take place during membrane fusion. In the first step, receptor binding, pH reduction and/or S protein proteolysis induces dissociation of S1 from S2. This step is documented for some MHVs. In the second step, the fusion peptide (FP) is intercalated into the host cell membrane. This is the fusion-intermediate stage. In the third stage, the part of the S protein nearest to the virus membrane refolds onto a heptad repeat 1 (HR1) core to form the six-helix bundle (6-HB), which is the final postfusion configuration of the S2 prot.