Medical Pharmacology Chapter 36:  Antiviral Drugs

• Viral Structure continued:

  • Viral Envelope Proteins:  Some but not all animal viruses have envelopes,  including those viruses exhibiting helical symmetry (influenza viruses and others) and many showing icosahedral symmetry (herpesviruses and others).¹

    • Envelope viral membranes include associated proteins.¹

    • These proteins are typically integral membrane proteins and many have been glycosylated.

      • Glycoproteins in virion envelopes often present as repeating units, described as multimeric.

        • One example, influenza A contains to glycoprotein species, one trimeric (hemagglutinin) and the second, a neuraminidase (tetrameric).

          • A nonglycosylated, additional protein species (M2) is also found.¹

    • Many membrane associated proteins are found to span the membrane, such as those associated with signal transduction e.g. G proteins.

      • However, envelope proteins may span the membrane only once although hepatitis B virus is an example of one spanning the membrane several times.¹

      • Envelope virus surface glycoproteins may assist in fusing virion membrane to cell membrane during infection.

      • Envelope viruses such as influenza viruses and retroviruses exhibit a protein layer between the envelope in the nucleocapsid.¹

        • This layer consists of "matrix protein" or M protein.¹

          • Influenza A virsus³

            "The influenza A virus particle has a lipid envelope that is derived from the host cell membrane. Three envelope proteins-hemagglutinin (HA), neuraminidase (NA) and an ion channel protein (matrix protein 2, M2) - are embedded in the lipid bilayer of the viral envelope. HA (rod shaped) and NA (mushroom shaped) are the main surface glycoproteins of Influenza A viruses. The HA glycoprotein is responsible for binding of the virus to sialic-acid residues on the host cell surface and for fusion of the viral envelope with the endosomal membrane during virus uncoating. The NA glycoprotein cleaves sialic-acid receptors from the cell membrane and thereby releases new virions from the cell surface. M2 functions as a pH-activated ion channel that enables acidification of the interior of the virion, leading to uncoating of the virion. Matrix protein 1 (M1), which is the most abundant protein in the virion, underlies the viral envelope and associates with the ribonucleoprotein (RNP) complex. Inside the M1 inner layer are eight single-stranded RNA molecules of negative sense that are encapsulated with the nucleoprotein (NP) and associated with three RNA polymerase proteins,  polymerase basic protein 1 (PB1), PB2, and polymerase acidic protein (PA), to form the RNP complex. The PB1, PB2 and PA proteins are responsible for the transcription and replication of viral RNA."3

          • The molecular structure of several matrix proteins which belong to the order Mononegavirales [respiratory syncytial virus (RSV), Ebola virus and Borna virus] have been determined.²

            • These structures exhibit one or two domains with similar β-sandwich stuctures.

            • Paramyxovirus M protein may form a grid-like structure on the viral membrane inner surface.

            • Paramyxoviruses include human pathogens affecting the respiratory system.

              • Newcastle disease virus (NDV) is an avian Paramyxovirus.

                • The NDV M protein shows >20% sequence identity with other Paramyxovirus M protein such as mumps, measles and the parainfluenza viruses.²

    • Other viruses not exhibiting this layer shows direct nucleocapsid interaction with integral membrane proteins.

  • Viral Lipids: 

    • The viral envelope contains lipid which is acquired during the process in which viral nucleocapsids bud through cellular membrane.

      •  

        Enveloped Virus Budding

        Attribution:  Harrison S Budding of Enveloped Viruses https://www.youtube.com/watch?v=JCd7JdAsvI8 (11/2013) iBiology Techniques YouTube Channel:  https://www.youtube.com/channel/UCimxROEIlNsiRqT9X5tg2XQ

      •  

        Viral Budding

        PhD_Dre Generic viral budding http://www.en.wikipedia.org/wiki/File:Budding_of_generic_virus,_pictorial_represent.jpg

    • This process occurs during viral maturation and the site of binding is determined by viral proteins inserted into the host cell membrane.^4,5

      • Virion membrane phospholipid composition is therefore determined in part by the lipid composition characteristics of the host cell.

      • An example of this process is found in the HIV-1 envelope; moreover, the virion membrane albeit derived from host cell membranes undergoes modification prior to incorporation.

        • In the HIV-1 case the viral envelope contains relatively more cholesterol and sphingomyelin compared to the plasma cell membrane but less phosphatidylcholine and phosphatidylinositol.

        • Another factor is which cellular membrane is used in the budding process.⁶ 

          • For instance, influenza virus buds outward from the cell surface and reflects cholesterol and phospholipid concentrations consistent with those proportions found in the plasma membrane.

            • By contrast, flaviviruses but into the endoplasmic reticulum and exhibits relatively lower cholesterol content consistent with the lipid composition of the endoplasmic reticulum membrane.

      • The viral genome is protected from the environment in part by the lipid bilayer which acts as a permeability barrier.

        • Therefore, enveloped viruses may exhibit reduced safety-margin with respect to protein assembly compared to non-enveloped viruses.⁶

      • Viruses that replicate in more than one kind of host cell will exhibit different envelope lipid compositions depending on the host cell type.

        • For example, the alphavirus Semliki Forest virus hosted by hamster kidney cells contain fivefold more cholesterol compared to virions produced in mosquito cells.

          •  

            Reconstruction of Semliki Forest Virus

            A2-33 Cryo-electron microscopy reconstruction of Semliki Forest virus at 9A resolution. December, 2013 http://www.commons.wikimedia.org/wiki/File:Emd-1017.jpg This virus was originally isolated from mosquitoes in the Semliki Forest, Uganda and causes disease in both man and other animals.  The virus may cause encepalitis in humans (mild symptons) http://www.en.wikipedia.org/wiki/Semliki_Forest _virus

        • Furthermore, acquiring the lipid-containing membrane is a central step in virion morphogenesis in some viruses.

          • Some lipid-containing viruses may be disrupted to the point of loss of infectivity by exposure to organic solvents such as ether.^4,5