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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