R E S E A R C H @ H K U S T
11
Unraveling Replication
When DNA replication goes wrong, the
consequences can be catastrophic: an
aborted fetus during the early stages of
development, deformed organs during
late-stage development, or cancer are
just some examples. The challenge
for scientists such as geneticist Prof
Bik-Kwoon Yeung Tye and life scientist
Dr Yuanliang Zhai is to understand
the fundamental mechanisms at work
in DNA replication in normal cells in
order to identify and correct problems
in diseased cells and stop the disaster
from happening.
The intricate replication process,
which only happens once in each
cell cycle, has proved highly difficult
to unravel, hampered by limitations
on what scientists could see of the
molecular machinery at work at the
atomic scale. But research by the Tye/
Zhai team, utilizing the latest cryo-EM
technology in collaboration with Prof
Ning Gao at Peking University, has
recently led to pioneering advances that
bring the previously unreachable into
the realms of the achievable.
Due to the higher resolution now
available from state-of-the-art cryo-EM
technology, the scientists were finally
able to solve one part of the puzzle: the
structure of the core of the MCM2-7
helicase enzyme at the near-atomic
level of 3.8Å. In accomplishing this
significant endeavor, it became the first
sub-nanometer structure reported for
the MCM2-7 complex. The complex
had been identified previously as
having a major role in destabilizing and
separating the double-stranded DNA
during replication. But it had defied
INSIDE
THE WORLD
OF DNA
REPLICATION
crystallization efforts so it had not
been viewed through the established
structural biology imaging technique of
X-ray diffraction.
The findings were published and the
impact analyzed in
Nature
in 2015. Some
of the major advances in the article
showed that: the two rings of a MCM2-7
complex form a tilted and twisted dimer
through the N-terminal domain; and
the central channel thus formed has
four constriction points that immobilize
duplex DNA for deformation.
It was also a moving moment for
Prof Tye, who in 1983 had published
the first paper to name and identify the
MCM2-7 complex as important in DNA
replication in eukaryotes, following
her research on yeast as an assistant
professor at Cornell University. “It
couldn’t have been more gratifying, as
I was literally and figuratively seeing
the fruits of the hard work of an entire
community that started from my own
lab,” she said.
Eukaryotes are organisms with a
complex cell that has its genetic material
stored within a membrane-bound
nucleus. They include fungi, plants,
and animals. Cellular mechanisms in
eukaryotes are evolutionarily conserved
and therefore similar in many ways,
enabling researchers to learn about
human diseases by studying yeast.
A major problem in preparing
samples of complex molecular machines
for studies is in keeping together the
large number of components that
make up these machines through
multiple steps of purification. Another
is the problem of scale. Dr Zhai, a
HKUST PhD graduate and currently a
research assistant professor, took up the
Each subunit of MCM2-7 forms constriction
points to immobilize DNA for deformation.
The tilted and twisted arrangement of two MCM single hexamers, the core of the
replication licensing complex, at the resolution of 3.8Å.
