The human herpes viruses is one of the most common DNA virus and is responsible for diseases such as chicken pox,
mononucleosis, lymphomas and Kaposi sarcoma.
To generate swarms of new viral particles, a virus hijacks a cell
into producing masses of self-assembling cages that are then loaded with
the genetic blueprint for the next infection. But the picture of how
that DNA is loaded into those viral cages, or capsids, was blurry,
bacterial viruses and human herpes virus.
‘New insight into two states of a viral protein reveal its function as a DNA-sensor and gives a roadmap for developing anti-viral therapy for pathogenic DNA viruses.’
researchers pieced together the three-dimensional atomic structure of a
doughnut-shaped protein that acts like a door or 'portal' for the DNA
to get in and out of the capsid, and have now discovered that this
protein begins to transform its structure when it comes into contact
with DNA. Their work published in Nature Communications
"Researchers thought that the portal protein acts as an inert
passageway for DNA," says senior author Gino Cingolani, a
Professor in the Department of Biochemistry and Molecular Biology at Thomas Jefferson University and researcher at the Sidney Kimmel Cancer Center.
"We have shown that the portal is much more like a sensor that
essentially helps measure out an appropriate length of DNA for each
capsid particle, ensuring faithful production of new viral particles."
The finding solves a longstanding puzzle in the field, and reveals a
potential drug target for one of the most common human viral pathogens,
Dr. Cingolani and colleagues started their work 18 years ago by
characterizing the structure of the portal protein using X-ray
crystallography from P22, a bacteria-infecting virus that has a nearly
identical portal protein as the human-infecting herpes viruses. In a
paper published in 2011, the group showed that the portal protein looked
like a perfect doughnut on a pedestal, with 12-fold rotational
symmetry. Paradoxically, though, the protein wasn't very good at binding
to DNA, which should have been an essential ability, the researchers
"We figured the portal protein we had studied for over a decade must
be an end-stage, or mature version, of a more plastic and dynamic
molecular machine," says Dr. Cingolani. "And that it must also adopt
other conformations earlier in viral assembly that have the capacity to
bind both DNA and other motor proteins, or terminases."
Proteins can change structure and hence their function and
biochemical activity many times before reaching their final mature
state. While these intermediate states are unstable and sometimes exist
for tiny fractions of a second, they can also have important functions.
In the current paper, Dr. Cingolani and colleagues describe their
success in identifying and characterizing an immature state of the
portal protein, of which they determined the three-dimensional structure
down to the atomic level. Unlike the mature final-stage protein that
was profoundly symmetric, this immature conformation of portal protein
is surprisingly asymmetric and has the ability to bind strongly to both
the motor and the DNA itself.
"We think that DNA binds to the immature portal protein and wraps
around it like a python, as it enters the viral capsid with the help of
the motor protein. This DNA stranglehold causes the portal protein to
begin to transform into its final symmetric state that because of its
weak binding will ultimately release both the DNA and the motor, cutting
off the DNA-loading at an appropriate length," says Dr. Cingolani.
"It's a completely novel mechanism for sensing DNA. It's a
conformational change from asymmetric to symmetric that's completely
unexpected, yet makes perfect sense."
In addition, the portal protein is unique to viruses, which makes it - in all of its various forms - a potentially good drug target.
Because some herpes viruses infect and lay dormant in human cells until
they reawaken by stress, developing a therapy that could interfere with
viral production at different levels could prove a useful therapeutic
"It took us 18 years to understand that the portal protein functions
by existing in two states that turn the viral DNA packaging on and off
by changing its structure. At 18, it feels like this story has come of
age along with the research," says Dr. Cingolani.