Retroviruses

Front 1

Structure of simple retrovirus

Back 1

Envelope
-single spike glycoprotein (EN)
-SU (Surface) antibodies bind to
-TM (transmembrane)
MA (matrix) protein under lipid bilayer and hold everything together
CA (capsid) protein forms shell around outside
Nucleic acid: 2 strands of + sense ssRNA tightly bound by multiple copies of nucleocapsid protein. 5' cap and 3' polyA tail like most eukaryotic mRNAs
Protease
Integrase
Reverse transcriptase

Front 2

Initial Phase by Virion-associated Enzymes

Back 2

Attachment
Penetration
Reverse transcription in capsid core – converts ssRNA to dsDNA
-Uses viral protein “Reverse transcriptase”
Transit to Nucleus
-Some Retroviruses require mitosis for nuclear breakdown (not HIV)
Integration into Host Cell Chromosome
-Uses Viral Protein “Integrase”
-Integrated DNA copy of virus genome is called a “Provirus”

Front 3

Second Phase Requires Cellular Enzymes

Back 3

Cellular Pol II synthesizes viral RNA
Viral RNA is processed into mRNA (splicing) and genomes
Synthesis of viral proteins
Assembly and budding
Proteolytic processing (maturation cleavages) within virion by viral protease

Front 4

mRNA splicing

Back 4

spliced mRNA is default pathway

sometimes unspliced mRNA is exported

Front 5

Basic Coding Structure of Simple Retrovirus Genome

Back 5

Group antigen (GAG)
Polymerase (POL)
Envelope (ENV)

Retroviruses make unspliced mRNA from the genomic + sense that includes GAG and POL

Spliced mRNA makes ENV

Front 6

LTR - Long Terminal Repeats

Back 6

“Promoter” region on provirus
Contains signals for initiation of transcription
Binding sites for cellular and viral transcription factors
Site of Recombinantion with host DNA during integration

Front 7

GAG

Back 7

MA – Matrix protein serves as “bridge” to link membrane spike proteins and internal capsid components.
CA – Capsid protein serves as inner shell surrounding the nucleocapsid
NC – Nucleocapsid protein serves to bind viral genomic RNA
PR – Protease protein is responsible for carrying out the cleavages in the gag and pol polyproteins

Front 8

POL

Back 8

RT – Reverse Transcriptase is an RNA-dependent DNA polymerase which converts
the input viral genomic RNA into DNA. This viral enzyme is incorporated into budding progeny virions.
IN – Integrase protein serves to recombine the new viral double-stranded DNA genome into the host cell DNA chromosome. The resulting inserted viral genome is called a “provirus.”

Front 9

ENV

Back 9

SU – Surface protein contains receptor-binding activity for virus

TM – Transmembrane protein is integral membrane protein with fusion activity.

TM and SU are synthesized as precursor; cleaved by cellular proteases. SU and TM are held together in complex.

Front 10

Two mechanisms for expression from the retrovirus genome

Back 10

1. GAG-POL is produced as a polyprotein by translation of viral genomic RNA.
-Two translation products: GAG and GAG-POL “Fusion”
-These two polyproteins are cleaved by viral protease
-Can get either GAG or GAG-POL due to frameshift at slippery site (slips only 1% of time)

2. ENV is produced by translation of an mRNA that is a spliced version of the viral genomic RNA

Front 11

Retrovirus Assembly

Back 11

0 – Env proteins (TM-SU) transported from ER, Golgi to plasma membrane.
Full length viral genome is synthesized by cellular polymerase. Forms a dimer in cytoplasm
1 – Dimer of RNA genome assembles with Env, Gag and Gag-Pol at plasma membrane
2 + 3 – Budding of virion
4 – Viral protease (PR) carries out maturation cleavages within virion
NOTE: small amounts of PR, RT and IN found in mature virion.

Front 12

Viral Oncogenes

Back 12

Oncogenes encode proteins which have the potential to transform cells.

Normal cell version is called c-oncogene or proto-oncogene.
-Examples: Growth factors, receptors for hormones, G-proteins, kinases, transcription factors

Viral oncogene can differ from a cellular oncogene:
-Contains only a portion of whole oncogene
-Often do not contain splice sites
-Separated from normal control signals and sequences

v-oncogenes can also result from fusions of two proteins (v-abl is fusion of gag and abl which now directs abl to cell membranes for transformation)

Front 13

Recombination Between Retrovirus and Host DNA
Can Generate Two Types of Oncogenic Retroviruses

Back 13

Transducing Oncogenic Retroviruses – genome encodes an oncogene that was “grabbed” from the host genome.
-Recombination between cellular “proto-oncogene” and
viral genome gives v-oncogene
-Viral genomes contain a cellular gene that becomes
oncogenic when expressed in viral context

Nontransducing Oncogenic Retroviruses –
-Recombination into genome next to cellular oncogene
-Transcription from viral LTR activates oncogene