8/9/2019 1-20-10V

1/18

January 20, 2010

8/9/2019 1-20-10V

2/18

Virus Structure

Capsid/Nucleocapsid Symmetry;

Viral Genomes

8/9/2019 1-20-10V

3/18

Major components of a virion

1. viral genome

comprised of DNA or RNA

2. capsid or nucleocapsid

capsid: protein shell surrounding the viralgenome in a virus particle

nucleocapsid: a nucleic acid - protein assembly

packaged

within the v

irion

3. envelope - host cell derived lipid bilayer containing

viral glycoproteins

the outer layer of many virus particles

8/9/2019 1-20-10V

4/18

- virion size

- virion shape

- capsid symmetry

- presence or absence of an envelope

examples: Fig. 2.1

Virion morphology:

8/9/2019 1-20-10V

5/18

Functions of v

irions

8/9/2019 1-20-10V

6/18

Constraints on v

iral caps

ids

8/9/2019 1-20-10V

7/18

Helical symmetry

Helical symmetry is described by:

- number of structural units per turn of

the helix (Q)

- the axial raise per structural unit (V)

- the pitch of the helix (P)

P = Qv V

8/9/2019 1-20-10V

8/18

Q = 16.3V = 0.14 nm

P= Qv V!2.3 nm

Tobacco mosaic virus (TMV)

Genome: ss (+) RNA

6.4 kb in length

Single coat protein

forms a helical capsid

Fig. 2.2

8/9/2019 1-20-10V

9/18

Icosahedral symmetry

Icosahedron is one of 5 regular solids with

regular polygonal faces20 triangular faces

12 vertices where 5 faces meet

30 edges where 2 faces meet

related by 2-, 3- and 5-fold symmetry

Closest available approximation of a

sphere

Capsid may form a hollow sphere structure

8/9/2019 1-20-10V

10/18

Icosahedral symmetry

8/9/2019 1-20-10V

11/18

The simplest icosahedral capsid

Five

ident

ical subun

its to form each of 12 vert

icesThree identical subunits to form each of 20 faces

the minimal number of subunits is 60

example: usedbyParvoviridaecapsidbuilt from 60 copies ofVP2

Fig. 2.3

Viruses with larger

genomes need largercapsids with more

subunits.

8/9/2019 1-20-10V

12/18

Major components of a virion

1. viral genome

comprised of DNA or RNA

2. capsid or nucleocapsid

capsid: protein shell surrounding the viralgenome in a virus particle

nucleocapsid: a nucleic acid - protein assembly

packaged within the virion

3. envelope - host cell derived lipid bilayer containing

viral glycoproteins

the outer layer of many virus particles

8/9/2019 1-20-10V

13/18

Viral Genomes

- replication of viral genome

- production of viral proteins

- assembly of proteins and genome into a

virion

- release of viral progeny from infected cells

- evasion of host defenses

- transmission to new (uninfected) hosts

Must specify proteins and signals for:

8/9/2019 1-20-10V

14/18

Structure and complexity of viral genomes

1. DNA or RNA-

2. linear or c

ircular-

3. segmented or non-segmented-

4. double-stranded or single-stranded-

-

8/9/2019 1-20-10V

15/18

Viruses with DNA genomes

- double-stranded with a few exceptions-

-

- size range from 3 kbp to 400 kbp-

-

- circular or linear-

-

-

8/9/2019 1-20-10V

16/18

Viruses with RNA genomes

- single-stranded with a few exceptions-

-

- size range from 6 kb to 33 kb

--

- segmented or non-segmented-

--

- exclusively linear

-

8/9/2019 1-20-10V

17/18

Viruses with RNA genomes

- positive (plus) - strand (sense)

-

- negative (minus, anti) - strand (sense)

-

- ambisense

-

Single- ordouble stranded:

8/9/2019 1-20-10V

18/18