HIV Replication

Rachel Carriger

Biochemistry Fall 2004

AIDS

• Acquired Immune Deficiency Syndrome

• First cases reported to CDC in 1981

• HIV-1 and HIV-2 viruses discovered in 1983 by Luc Montagnier

• HIV is classified as a retrovirus

HIV is a RetrovirusRetrovirus

Characteristics• Complex interactions

with the host cell• Chronic course of disease• Long and variable

incubation periods• Persistant viral

replication• Destruction of

immunological cells

HIV Characteristics• Chronic course of

disease• 5-10 year latency

period• CD4+ T lymphocytes

are the host cells• Replicates rapidly• Neurological

abnormalities

HIV structure• 72 glycoprotein

complexes on lipid membrane

• Gp120 and gp41 transmembrane protein

• Inside p17 matrix and p24 core antigen protein

• Two copies of RNA• Reverse transcriptase

enzyme• Nucleoprotein p7

HIV Entry Into CD4+ Host Cell

Gp 120 molecule attaches to CD4 glycoprotein site and chemokine receptor

GP 41 inserts its NH2 head into the membrane of the host cell

Reverse Transcriptase

• Discovered by David Baltimore and Howard Temin in 1970

• Purpose: to convert single stranded RNA into double stranded DNA

• Heterodimer of p51 and p66 subunits• p66 subunit consists of five subdomains:

fingers, thumb, palm, connector, and RNase H

Replication

• tRNA primer bound to 3’ terminal end• Binds free nucleotides at a rate of 20 per second

in the 5’ 3’• DNA/RNA hybrid formed• Ribonuclease H subdomain of reverse

transcriptase digests the original strand of RNA and leaves a string of purines to be used as primer for the new DNA strand

Reverse Transcriptase Conformational Changes

1. Fingers close down on the palm and hold the template strand in place

2. Fingers bend back releasing a pyrophosphate and base

3. Enzyme positioned at the 3’ end of the growing DNA chain so the replication process can continue

Transcription DNA RNA

• HIV incorporates itself into genome of the host cell

• Transcription factor NF-kB binds to LTR regions of DNA

• Regulatory proteins rev and tat produced first

• New RNA strand exits cell by budding

Gene Sequence of HIV

• Gag- group antigen, nucleus

• Pol- polymerase, enzyme

• Env- envelope, outer membrane

Difficult To Find A Cure• Reverse transcriptase has a high mutation

rate – Very tolerant of non standard base pairs– No exonuclease activity to proofread– 1-10 errors per genome per replication cycle

meaning 109 new viruses a day

• Replicates very rapidly• Long latency period• Drugs targeted at reverse transcriptase and

protease

Protease Inhibitors

• HIV protease (aspartic protease) cleaves polypeptides that the virus makes and needs for maturation

• Protease Inhibitors bind tightly to the HIV protease and keep it from functioning

Nucleoside Inhibitors

• Analogs of common base pairs used during replication

• Slight modifications in structure stop the whole replication process

Non-Nucleoside Inhibitors

Binds to reverse transcriptase and alters it’s structure

Nevirapine Delavirdine