Cell Bio Retroviruses

general structure of a retrovirus particle

consists of an RNA genome packaged in a protein capsid, surrounded by a lipid envelope

lipid envelope

contains polypeptide chains including receptor binding proteins that link to membrane receptors of host cell, initiating infection

what do retroviruses use as hereditary material?

RNA in place of DNA

retroviral genes and their functions

• GAG: group-specific antigen; gene of structural proteins

• POL: polymerase gene; codes for protease, RTase, and integrase

• ENV: envelope gene; codes for membrane proteins gp41 and gp120

LTR

in viral DNA, genes are bracketed by long terminal repeats, identical sequences that can be divided into 3 elements, including a promoter; these sequences are needed for successful transcription

sequence that every retrovirus contains

LTR-GAG-POL-ENV-LTR

reverse transcriptase (RTase)

• makes a DNA copy of the viral RNA molecule

• acts as a nuclease to remove the RNA and makes a second DNA strand

integrase

integrates newly synthesized viral DNA into host chromosome

enveloped viruses

• protein shell is further enclosed by outer lipid bilayer membrane

• envelope contains proteins that enable the virus to bind to cells and aid its entry into the cell

retrovirus life cycle (4 steps)

• begins in nucleus of infected cell where retroviral genome is a DNA element integrated into DNA of host cell

• genome of virus is ~8-12kb of DNA

• full-length genomic mRNA is made initiated at beginning of the repeat at the 5’ LTR

• free particle can now infect new cells by binding to cell surface receptor

retrovirus replication (4 steps)

• after infecting a cell, RTase is used to make initial double-stranded copies of viral DNA that’s inserted into host cell chromosome

• host cell RNA polymerase is used to make virus RNA that serve as templates for making new copies of viral RNA and also serve as mRNA

• mRNA is translated into viral proteins used to make virus envelope

• new viral particles are assembled, bud from PM, and are released

what type of cell does HIV target?

subset of T lymphocytes

what is the receptor that HIV binds to called?

CD4

RV protein processing

• viral mRNA leaves nucleus; translation of viral mRNA results in synthesis of 3 polyproteins: ENV gp 160, GAG p55, and GAG-POL p 160

• ENV gp 160 proteins pass through ER and Golgi to be processed into gp120 and gp41 HIV envelope proteins

• during movement through Golgi, glycosylation of gp120 occurs

ribosome frame shifting

generates p55 and p160 from the same mRNA strand

RV budding

• gag and gag-pol polyproteins associate with inner surface of PM and interact with gp41

• as p55 and p160 accumulate on inner surface of PM, they aggregate and commence assembly to form the virion

• as assembly continues, structure extrudes from cell

RV maturation

after budding, viral proteinase in p160 becomes active, causing the cleavage of p160 and p55 into various subunits and generating mature form of HIV

what can be cut out and what can stay when RVs are used as expression vectors?

• viral genes are cut out

• packaging sequence (ps) is kept

what genes/marker are commonly put in RV vectors?

lacZ marker enzyme and GFP