November 26, 1998
Proteins Harbor A Shocking Tale of Evolution
A team of scientists from the Howard Hughes Medical Institute at the
University of Chicago has found that a ubiquitous protein may explain
how relatively sudden changes in body shape occur in a species.
Working with fruit flies, HHMI investigator Susan Lindquist and
colleague Suzanne L. Rutherford found that under normal circumstances,
heat shock protein 90, Hsp90, suppresses genetic signals that can alter
body shape. Heat shock proteins are induced by stress, and they confer
protection against heat, oxygen shortages and free radical damage. When
the level of Hsp90 in cells is reduced by half, which mimics what can
happen when animals undergo stress, malformed wings, eyes, legs and
antennae develop. Astonishingly, the experiments showed that the
deformed flies survived, bred and passed on these variations to
subsequent generations that have normal Hsp90 function.
“Hsp90 appears to be a quick route to adaptation. It provides a reservoir for change, for exposing variation.”
Susan Lindquist
In the November 26, 1998, issue of the journal Nature,
Lindquist and Rutherford suggest that Hsp90 enables genes to store
instructions for sudden structural change. Such rapid changes may occur
when a species' environment changes drastically and a slightly
different body shape might provide a better chance of survival.
Initially, the variations observed by Lindquist and Rutherford arose
in only 1 to 3 percent of a given population of flies. By selectively
breeding the deformed flies, however, the researchers demonstrated that
in just a few generations, 80 to 90 percent of the progeny exhibited
the deformity.
Many heat shock proteins double as molecular "chaperones" that
escort proteins through the process of folding into their final
three-dimensional shape. Hsp90 is one of the most abundant chaperones,
but little is known about its function beyond the fact that it
interacts with proteins essential for cell proliferation and embryonic
development.
"Hsp90 seems to help a lot of these proteins when maintain a stable
state," Lindquist said. "It's like a support function. Basically, its
job is to take care of other proteins that tell a cell what it is
supposed to be doing. These proteins are often unstable because they
need to change their shapes in response to the various signals that
regulate growth and development."
Lindquist and Rutherford found that when they increased the
temperature of developing fly embryos, many more deformed flies were
born. Moreover, a similar range of deformities was observed when the
flies were fed a chemical that blocks Hsp90 function.
Lindquist and Rutherford believe their experiments suggest that
environmental stress may uncover pre-existing genetic variation that
provides a means for animals to quickly adapt to shifts in environment.
If so, Hsp90 would be the first such molecular mechanism that underlies
drastic morphological change rather than the small, progressive change
that is known to occur in evolution.
This flood of morphological change may help explain the great flurry
of diversity unleashed on Earth about 570 million years ago during the
Cambrian period. According to the fossil record, dozens of new animals
in astounding shapes and sizes first appeared during the Cambrian
period.
"Hsp90 appears to be a quick route to adaptation," Lindquist says.
"It provides a reservoir for change, for exposing variation."
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