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 appears to be a quick route to adaptation. It provides a reservoir for change, for exposing variation.”
In the November 26, 1998, issue of the journal
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."