Scientists from the National Centre for Biological Sciences (NCBS), Bangalore, have discovered how the Herpes virus escapes one of the defense mechanisms it encounters in its hosts. Cells infected by Herpes viruses bind the viral DNA with proteins called PML NBs (ProMyelocytic Leukemia protein Nuclear Bodies) to stop the production of viral proteins and stall virus reproduction. The viral DNA, however, escapes from its protein prison with the help of ICP0 (Infected Cell Polypeptide 0), a viral enzyme. Although this has been known for quite some time, the mechanisms behind the viral Houdini act were unclear. Now, NCBS researchers have found that the Herpes virus uses ICP0 to exploit the host cell’s machinery to tag PML NBs for destruction and free viral DNA. A unique crosstalk mechanism between protein modifications was discovered to be responsible for this process.
When a Herpes simplex virus enters a human host cell, a number of defense systems are activated to stop the viral invasion. One of these defenses uses proteins called PML NBs (ProMyelocytic Leukemia protein Nuclear Bodies) to bind viral DNA within a cage-like structure and stall the production of viral proteins and virus reproduction.
The Herpes virus, however, does a Houdini act and slips its protein cage. Although the viral protein ICP0 (Infected Cell Polypeptide 0) and several host cell proteins were known to be involved in this breakout, the mechanism of how the escape happens remained a mystery.
Now, Ranabir Das’s group from the National Centre for Biological Sciences (NCBS), Bangalore, has discovered how the Herpes virus breaks out of its protein jail. The virus uses host cell proteins to turn ICP0 into a potent enzyme that targets and tags PML NBs for destruction. The work, published in the Journal of Molecular Biology, demonstrates that the Herpes virus exploits three forms of protein modifications to help its DNA escape the PML NBs.
Protein modifications are akin to switch controls that cells use to modulate protein function. Proteins can be activated, deactivated, ferried to different parts of the cell, or adjusted to even take on entirely new functions by the addition or removal of molecules such as sugars, fats, or phosphates. Sometimes, these modifications are complex—involving tagging with other small proteins such as ubiquitin or SUMO (Small Ubiquitin-like Modifier). For example, the cell often tags unwanted or damaged proteins with ubiquitin molecules; the ubiquitin-tags ‘mark’ these proteins as targets for the cell’s recycling systems.
The viral protein ICP0 happens to be an enzyme that can add ubiquitin tags specifically to SUMO-carrying proteins such as the PML NBs; this ubiquitin-adding function becomes more potent when ICP0 is phosphorylated at a specific site. Through extensive molecular experiments, Das’s group has identified this site on ICP0 – dubbed SLS4 – through which the protein binds to PML NBs. Phosphorylation at SLS4 increases ICP0’s affinity for binding the SUMO tags on PML NBs, which in turn, is also responsible for ICP0 becoming a potent ubiquitination enzyme for PML NBs. The ubiquitin tags added by ICP0 mark the PML NBs for destruction, thereby freeing the viral DNA.
But how does ICP0 become phosphorylated at SLS4 in the first place? Turns out that this too, is mediated by the virus. It is well known that the Herpes virus activates the host cell protein Chk2 in order to arrest dividing cells during a specific stage of cell division; this arrest helps virus growth and reproduction. Das’s group has now found that Chk2 also plays a second role in helping the virus – it phosphorylates ICP0 at SLS4.
“Our study provides the atomic details of how the virus disrupts and degrades the cage of nuclear protein structures which the host cell has designed to subdue the virus,” says Ranabir Das. “What’s more, this work uncovers a novel crosstalk between the ubiquitin, SUMO, and phosphorylation signalling systems which the virus exploits to escape the host’s defences,” he adds.
Such molecular crosstalk is singularly unique; it has never been observed before in any system.
“Since the work aimed to provide detailed molecular information on how ICP0 is phosphorylated and attaches to the PML NBs, data collection was quite a challenge. We had to solve multiple high-resolution NMR structures, carry out intense biochemical studies and virology experiments. Going back-and-forth between these multiple experiments was the toughest part of the work,” says Das.
“Herpes infections in new-borns can be fatal, and in people with compromised immunity, recurrent infections can cause severe problems. Also, there are currently no vaccines against the herpes simplex virus. Understanding the molecular mechanisms at play during the viral life-cycle is therefore vital,” says Das to highlight the importance of this work. “The information that our study provides will be very useful in designing antiviral therapies to combat Herpes infections,” he adds.