HIV Uses Small Molecule To Strengthen Its Protective Capsid

The human immunodeficiency virus (HIV) hijacks a small molecule from the host cell to protect itself from being destroyed by the host’s immune system, scientists from University Of New South Wales, Sydney and the UK have discovered. They identify a new target for antiviral therapy against HIV and provide a method for testing and measuring new drugs designed to target the capsid.

HIV forms a protein shell – called a capsid – that shields its genetic material from host defence mechanisms as it enters the cell and makes its way to the nucleus to establish infection.

Using a new single-molecule microscopy technique – developed at UNSW’s Single Molecule Science in the Faculty of Medicine – the research teams found that HIV specifically incorporates a small molecule from the host cell – inositol hexakisphosphate – to strengthen its capsid. The host inadvertently provides the key for the virus infecting it to lock down the protective shell, keeping the genetic cargo safe until it is released into the nucleus.

Kinetics Of HIV-1 Capsid

UNSW PhD student Chantal Márquez is involved in the studies and is the first author of the paper describing the new method.

“The HIV capsid falls apart within minutes once it’s isolated from the virus. Our strategy lets us study exactly how a native capsid breaks apart in real-time without taking it out of the viral membrane,”

said Associate Professor Till Böcking, who led the UNSW team involved in both studies.

ATP binds to the HIV-1 capsid

ATP binds to the HIV-1 capsid.
Credit: Donna L Mallery, et al. CC-BY

With the help of Associate Professor Stuart Turville of the Kirby Institute, the team engineered viruses with fluorescent tags to monitor the viral capsid using fluorescence microscopy.

“We can now see the effect of different molecules on the capsid, and pinpoint precisely when it cracks open and begins to collapse,”

said Associate Professor Böcking.

Inositol Hexakisphosphate

The researchers found that inositol hexakisphosphate, which is abundantly present inside mammalian cells, makes the capsid much stronger, stabilizing it for 10-20 hours.

“It’s like a switch. When you bind this molecule, you stabilize the capsid, and release the molecule to open it up,”

explained Associate Professor Böcking.

“Capsids need to be much more stable inside a cell because the infection process takes hours, not minutes – so we wanted to find out what keeps it stable inside a cell,”

said Dr David Jacques of Single Molecule Science, who is an author of both studies.

“The HIV capsid has been intensively studied, but the question of how it can simultaneously be both stable and poised to ‘uncoat’ has been one of the great unanswered questions in HIV biology,”

said Dr Leo James, leader of the research team at the Medical Research Council Laboratory of Molecular Biology in Cambridge, UK.

Most of the currently approved HIV therapies target enzymes needed at different stages of the virus’ life cycle, but none of them are directed at the HIV capsid. New drug alternatives could improve the treatment of HIV with reduced toxic effects.

Donna L Mallery, et al.
IP6 is an HIV pocket factor that prevents capsid collapse and promotes DNA synthesis
eLife 2018;7:e35335 DOI: 10.7554/eLife.35335

Chantal L Márquez, et al
Kinetics of HIV-1 capsid uncoating revealed by single-molecule analysis
eLife 2018;7:e34772 DOI: 10.7554/eLife.34772

Top Image: Inositol hexakisphosphate fits into the pore structures (pink) of the HIV capsid (teal), and makes the makes the capsid stronger. Credit: UNSW