Structural Properties of viruses

1. The Structural properties of viruses :
Capsids, Nucleic acids and envelope.
Structure of T4 bacteriophage, TMV
and HIV.
Dr. R. S. Jadhav
Department of Microbiology,
VNBN Mahavidyalaya, Shirala.

2. Introduction
 Term virus is a latin words for “slimy liquid” or “poison”.
 Ivanowsky was discoverd first virus TMV (Tobacco mosaic virus) in
1892.
 Virus are acellular infectious entities whose genomes are either RNA or
DNA.
 Viruses multiply only in their specific host cell.
 The viruses are obligate intracellular parasites, capable of synthesis only
within host cells.
 The viral host includes: Bacteria, Fungi, Algae, Protozoa, Plant and
Animals.
 Viruse have two different state: 1) Extracellular and 2) Intracellular
state.
 1) Extracellular state-Virus is an inert infectious particle or virion.
 2) Intracellular state- Virus is a very active particle also replicating and
synthesizing its components using host cell biosynthetic machinery.

3. Characteristics of viruses
 Viruses are ultramicroscopic and can observed only in Electron microscope.
 They are very small and are measured in nanometers, which is one-billionth of a
meter. Viruses can range in the size between 17nm (Porcine circovirus (PCV) to
750 nm (Mimiviruses and Pandoraviruses)
 Most viruses vary in diameter range from 20 nanometres (nm; 0.0000008 inch) to
250–400 nm; the largest, however, measure about 500 nm in diameter and are
about 700–1,000 nm in length, which is 45,000 times smaller than the width of a
human hair.
 The basic structure of a virus consist a genetic molecule and a protein layer that
protects that genetic material. More complex viruses have additional layer is
enveloped containing lipids and carbohydrates.
 They can be cultivated only in living cells.
 Viruses are acellular particles, don’t have cell organelles like as cell wall,
cytoplasmic membrane, flagella, pili and ribosomes.
 Viruses metabolically inert particle because don’t have independent metabolism.
 They contain only one type of nucleic acid either DNA or RNA.
 Viruses do not grow and divde. However, they produce new virus particles by
using biosynthetic machinery.
 Many smaller viruses can crystallized and behaves like chemicals.

4. Viruses consider as link between livings and
nonlivings
 They contain nucleic acid as their genetic material.
 They multiply in living cells using cell biosynthetic
machinery.
 They undergo mutation.
 Viruses are thermo sensitive and are inactivated within
seconds at 56 o C.
 Sensitivity to physical and chemical agents. UV rays and
ionizing radiation inactivate them. The iodine, chlorine
and formaldehyde are actively virucidal.
 They are infectious and cause a number of diseases in
plant, animals and humans.
 At present study, viruses have been responsible for 8
human cancers.

5. The Structural properties of viruses : Capsids, Nucleic
acids and envelope.
Capsids
Capsid is the protein coat surrounding the nucleic acid.
• Basically virus structure depends on the capsids
• Simple viruses have only two layer (NA+ Protein coat)
Eg. TMV.
• Some complex viruses have complex nucleocapsids
(NA+Proteins layers+ Protein coat).
• Complex viruses have lipoprotein layer covered
nucleocapsids (NA+Proteins layers+ Protein coat+ Envelope)
Eg. HIV

6.  The capsids have highly ordered architecture and exhibit symmetry
 Three types of symmetry- 1) Icosahedral (Polyhedral),
2) Rod/Cylindrical (Helical) and
3) Complex (Polyhedral+ Helical)
 1) Icosahedral Capsids- Icosahedral capsids are regular polyhendrons with 20
triangular faces and 12 corners. An exhibits axes of 2-fold, 3-fold and 5-fold
symmetry passing through its edges, faces and vertices (corners) respectively.

7. •In icosahedral capsids triangular face are present in multiplies of 20,
whereas protein appendages at vertices are present in multiples of 12.
•The most economical symmetrical shell of maximum internal volume
with non-symmetrical protein molecules is an icosahedral.
Icosahedral capsid size is fixed by its geometry.
•The capsid surface is usually rough and size determines genome size.

8.  The basic structural unit of a capsid is called as capsomers. Capsomers can be
made up of several proteins, each of which is a protomers.
 Some viruses have only one protein in their capsids, the protomer and capsomer
are equivalent.
 Viruses have two protein in their capsid, the protomer and the capsomer are not
equivalent.
 The protomer are two types : Pentamers and hexamers. Pentamers have five
subunit and present at corners (vertices) and hexamers have six subunit and form
edges and triangular faces of an icosahedron.
 Table: The number of capsomers in capsids in different viruses.
Sr. No. Viruses Capsomers Numbers
1 PhiX174 12
2 Poliovirus 32
3 Togavirus 32
4 Polyoma Virus 72
5 Papilloma virus 72
6 Herpes virus 162
7 Adenovirus 252
8 Tipula iridescent virus 812

9. HIV (Human Immunodeficiency Virus)
 HIV is causative agent of Aquired Immunodeficiency Syndrome(AIDS).
 HIV belongs to Retroviridae family and infect to human immune system such as
CD4 and macrophages. HIV particle enveloped, icosahedral structure is present.
 Size of virus is measuring 100 to 120 nm in diameter. The envelope consist of
plasma membrane that is derived from the host-cell membrane.
 In addition to host membrane the envelope contains virus encoded glycoprotein
gp160 (spike or peplomers). Each particle have 72 glycoprotein and made up of
gp120 and gp 41. The gp120 is covalently attached to gp 41.
 Fig: Structure of HIV

10.  The gp41is embedded in plasma membrane and gp120 come out as
projection and act as the viral anti-receptor for attachment of host cell
receptor.
 MA(matrix protein) is made from the protein 17 and is present just
below the (lipid layer) envelope.
 Conical shaped capsid protein is made from the P24.
 There are three enzyme is present in core region, these are reverse
transcriptase, integrase and protease.
 Two identical copies, positive sense SS RNA molecule contain 9500
nucleotide.

11. 2) Helical capsids (Rod shaped Viruses)
 The capsomeres are arranged in a helix around a single rotational axis.
 The capsomeres curve into a helix because they are thicker at one end
than the other.
 The capsomeres bind non covalently to genome in periodic fashion, and
size of the genome determines length of capsid. Helical viruses may have
naked capsids eg, TMV and M13, or enveloped capsids eg.,Influenza,
Mumps and Measles virus.
 Most RNA viruses have helical symmetry are enveloped.
 Fig: Viral families representing helical capsids

12. TMV (Tobacco mosaic virus)
 The TMV is a plant virus that infects tobacco plant and causes mosaic
patterns (mottling and discoloration) on leaves.
 TMV is a rod shaped virus of 300 nm by 18nm diameter. The central
opening along the axis has a diameter of 4nm.
 Fig: Structure of TMV
 The MW of the TMV virus particle is 40 X 106 d.
 The capsid consists of 2130 identical capsomers.

13.  Each capsomer is made up of 158 amino acid residues with a MW of
17,300 d.
 The capsomer are arranged in a helix around central opening.
 TMV consist of SS RNA.
 There are about 6395 nucleotides in the RNA and MW of 2.1X106. The
RNA is arranged in a helix. Each turn of RNA helix contains 49
nucleotides and 16.3 capsomers are attached to RNA per turn of the
helix. Thus 3 nucleotides are linked to a single capsomer.

14. 3) Complex capsids
Some viruses have complex capsids.
 Bacteriophages (bacteria eating viruses) of the T- series are example of complex capsid
viruses.
 T-series phages include T1 to T7 and all are coliphages because they infect coliform
bacteria.
 Out of the 7 phages, 3 are even number phages and four are odd number phages. T- even
phages (T2, T4 and T6) have tadpole like shape with head and tail regions. These phages
infect E.coli.
 T-odd coliphages (T1, T3, T5 and T7) do not contains contractile sheath, T1 and T5
contains sheathless and baseplateless tail with rudimentary tail fibers. T3 and T7 contain
tailfiberless, short and noncontractile tail.
 The T4 infect nonmotile strain B of E.coli. The T4 phages particle consist of naked
icosahedral head and helical tail.
 The size of head is 95 X 65nm. It consist of about 2000 identical capsomers. 50µm long
double stranded DNA is tightly packed into the head. The DNA is circular and terminally
redundant. It contains an unusual base 5-hydroxylmethyl cytosine (5-HMC). All T- even
phages contain 5-HMC where as T- odd phages don’t contain 5-HMC.
 T4 phage contain of a long helical tail. The tail is attached to head with a holder called
coller.
 Whiskers are attached to coller, which hold tail fibers around tail. The tail tube has size of
80X18nm.
 The tail consist of a hollow tail tube with an internal hole of 2.5nm. Tail tube surrounds a
contractile protein sheath.

15.  The sheath is made up of 24 rings, each with 6 subunits. Thus there are 144 subunits in the
sheath.
 The sheath is connected to the collar at the upper end and to a base plate at the lower end.
The base plate is hexagonal and has a pin at each corner. The plate helps in adsorption and
penetration of the phage to its host.
 There are reports that the lysozyme like enzymes are associated with the plate. A long tail
fibers is given out from each of the six corners. The fibers are 130X2nm in size and they
help the virus to attach the host cell receptors.
 The capsid is significant in several ways. It condenses and confines genome and enzymes. It
protect the viral genome from physical chemical or enzymatic damage.
 It determines if a host cell is suitable for infection. It start the actual infection by attaching
and penetrating viral genome into the host cell.
 Structure of T4 Bacteriophage

16. Nucleic Acid
 Viruses contain only one type of nucleic acid either DNA or RNA.
 Viral nucleic acid may be single stranded or double stranded, linear or circular,
segmented or unsegmented and some have nick in their genomes.
 There are four types of nucleic acids based on number of strands:
 Single-stranded DNA
 Double-stranded DNA
 Single-stranded RNA
 Double-stranded RNA
 All these four types are found in animal viruses.
 The double-stranded DNA viruses and single-stranded RNA viruses are most
abundant. Most of the plant viruses have single-stranded RNA as genome and
bacteriophages contains double-stranded DNA as their genomes.
 Some virion DNA molecules such as Adenoviridae, P arvovirus, Lambda, T2,
T4 and T7 have special type of termini……….
 Cohesive ends/Sticky ends - the single stranded complementary projections at
5’ ends on DS DNA that can stick with each other to form circular molecule.
The Lambda phage contains cohesive ends.

17.  Terminal inverted repetition- TIR refer to the identical sequence present in
reverse orientation at the end. Adenoviruses, P arvoviruses and transposons
contain terminal inverted repetition.
 Terminal redundancy- It describes the repetition of the same sequence at both
end of DNA eg., ATGCATGC the terminal redundancy sequence can be
ATGCATGC. Terminal redundancy is seen in some phages such as T4, T2 and
T2.
 Circular Permutation- Some phages T2 and T4 show circular permutation.
When each virion DNA of a phage contains different set of DNA sequences at
ends, it is referred as circular permutation. Eg., if genetic information is
ATGCATGC then circular permutation would be generate molecules
TGCATGCA,GCATGCAT, CATGCATG and ATGCATGC so on……

18. Envelope
 A membranous covering around nucleocapsid is called enveloped. Many animal viruses ,
few bacteriophages and few plant viruses are surrounded by envelope.
 The envelope is about 10 to 15 nm thick.it is flexible than nucleocapsids, thus exhibit
pleomorphism. The envelope is derived from the host cell and acquired by the virus as it
matures and emerges from the cell.
 Envelope consist of phospholipid bilayer in which proteins are present ant virus specific
glycoproteins are inserted. Based on source of membrane there are two groups of viruses
 : I) ss RNA viruses and herpes viruses and
 II) PM2, ɸ6, iridescent insect virus and Pox viruses.
 I) the ssRNA viruses such as Togaviruses, Corona viruses, Retroviruses, Rhabdoviruses,
Bunyaviruses, Arenaviruses, Orthomyxoviruses and Paramyxoviruses multiply and assmble
in the cytoplasm of the host cell.
 These viruses release by a mechanism termed as budding through plasma membrane and
thus acquire envelope. Before budding viruses incorporate their own protein and
glycoproteins is destined to form viral envelopes.
 The virus glycoproteins that organize as spike like structures are called peplomers. The
peplomers usually project from the envelope they are clusters of viral encoded
glycoprotein(s) that are transmembrane in nature.
 The envelope in most cases contains plasma membrane. In some cases the membrane is
derived from ER(Endoplasmic reticulum) membrane (Ex. ,Arenaviruses), the Golgi
membrane (Ex., Bunyaviruses), and Nucleare membrane (Ex., Herpesviruses).
 The lipid composition of the viral envelope similar to the lipid composition of the host cell
membranes.
 The size (4 to 20nm) and shape (spike/club/petal) of peplomer vary from one animal virus
to another.
 In some viruses such as Orthomyxoviruses and Herpesviruses there are more than one type of
peplomer.

19. II) PM2, ɸ6, iridescent insect virus and Pox viruses.
 DS DNA bacteriophage, PM2, dsRNA bacteriophage ɸ6, ds DNA iridescent insect virus and
Pox viruses synthesize their own envelope membrane.
• Envelope membrane essential for infectivity and stability of the envelope is
critical for transmission of virus. Envelope viruses contain lipids and are sensitive
to bile and lipid solvents. Few enveloped viruses stable in GI tract or dry non-
aqueous environments.
 Attachment: Viral peplomers play role in attachment of virus particles to specific receptors
on the RBCs leading to hemagglutination.
 Attachment and Penetration: Viral peplomers play role in attachment and penetration of
virus particles to specific cell. Some peplomer is activated after cleaving it with host
protease. The mumps virus contains HN and F two type peplomers. In some cases a single
peplomer mediates attachment and penetration.
 Virulence factor: In mumps and influenza virus nuraminidase is present on one of the
peplomers. The nuraminidase digest sialic acid (Mucus of upper respiratory tract) and
allows the virus to reach the epithelial cell surface. Thus nuraminidase contributes to
virulence.
 Fusion: Cause some viruses to induce infected cells to fuse adjacent uninfected cells and
form giant multinucleate cells.
 Hemolysis: Some peplomers interact with RBCs and lyse them.
 Maturation: Peplomers triggers envelopment and thus maturation.