Clays May Have Aided
Formation of Primordial Cells
©2003
Howard Hughes Medical Institute
4000 Jones Bridge RoadChevy Chase, MD 20815-6789
"We are not claiming that this is how life
started," said HHMI investigator Jack W. Szostak. "We are saying that
we have demonstrated growth and division without any biochemical
machinery."
October 24, 2003— Howard Hughes Medical Institute (HHMI)
researchers have discovered that clays may have been the catalysts
that spurred the spontaneous assembly of fatty acids into the small
sacs that ultimately evolved into the first living cells.
HHMI investigator Jack W. Szostak and colleagues
Martin M. Hanczyc and Shelly M. Fujikawa at Massachusetts General
Hospital also demonstrated that these vesicles could be induced to
grow and to split into separate vesicles under laboratory conditions.
They reported their studies in the October 24, 2003, issue of the
journal Science.
Szostak and his colleagues were prompted to perform
their experiments by the earlier work of other researchers who had
found that clays could catalyze the chemical reactions needed to
construct RNA from building blocks called nucleotides. They reasoned
that if clays could foster the formation of vesicles, it would not be
inconceivable that clay particles that had RNA on their surface could
end up inside such vesicles. If that were true, the result would offer
conditions amenable to the eventual evolution of living cells that
could self-reproduce.
“Other researchers had observed that if fatty acid
micelles, which are stable at basic conditions, are exposed to more
acidic conditions, they spontaneously assemble into vesicles,” said
Szostak. “This reaction has a long lag period, and some sort of
nucleation surface is required to trigger the process. We reasoned
that if the right kind of mineral surface was present, this lag phase
would be eliminated.”
In their experiments, Szostak and his colleagues
found that adding small quantities of the clay, montmorillonite, to
fatty acid micelles greatly accelerated the formation of vesicles.
They also discovered that many other substances with
negatively-charged surfaces also catalyzed formation of vesicles.
When the researchers loaded montmorillonite
particles with a fluorescently labeled RNA and added those particles
to micelles, they detected the RNA-loaded particles inside the
resulting vesicles. And, going a step further, Szostak and his
colleagues showed that when they encapsulated labeled RNA alone inside
vesicles, it did not leak out.
“Thus, we have demonstrated that not only can clay
and other mineral surfaces accelerate vesicle assembly, but assuming
that the clay ends up inside at least some of the time, this provides
a pathway by which RNA could get into vesicles,” said Szostak.
However, he said, even primitive, non-living,
cell-like structures need a mechanism to grow and divide. Thus, the
scientists explored the behavior of vesicles to which micelles had
been added — finding that acidic conditions induced the micelles to
become unstable and somehow incorporate themselves into a growing
vesicle.
“After we showed that efficient growth was
possible, the next problem was how to complete the cycle by persuading
these vesicles to divide,” said Szostak. The scientists discovered
that if they extruded larger dye-containing vesicles through smaller
pores, the result was a proliferation of smaller vesicles, which still
contained dye.
“Exactly how this proliferation happens is not
clear, and there are different models for the processes,” said Szostak.
“The important thing is that it all works. You end up with small
vesicles in which the contents stay mostly inside. This is important
if the process is to be vaguely analogous to biological cell
division,” he said.
“Now that we have a proof-of-principle that growth
and division is possible in a purely physical-chemical system, we are
working on a way to get this cycle to function in a way that is more
natural,” said Szostak. “Clearly, there are a lot of complicated and
interesting processes going on here, and how this pathway leads to
biological systems is not at all straightforward.
“We are not claiming that this is how life
started,” emphasized Szostak. “We are saying that we have demonstrated
growth and division without any biochemical machinery. Ultimately, if
we can demonstrate more natural ways this might have happened, it may
begin to give us clues about how life could have actually gotten
started on the primitive Earth.”
In particular, said Szostak, further research
should aim to demonstrate that the formation of RNA or a related
polymer molecule could occur concurrently with vesicle replication.
“Ultimately, we'd like to put them together and have replicating RNA
inside a replicating vesicle,” said Szostak. “If we could demonstrate
both processes under arbitrary laboratory conditions, we could begin
to work toward making them work under more and more natural
conditions.”
©2003 Howard Hughes Medical Institute
4000 Jones Bridge RoadChevy Chase, MD 20815-6789
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