Mussels Stick-on Protein Isolated
By Sean Henahan, Access Excellence
 Wilmington,
Delaware (9/19/97)- Researchers have isolated the protein producing
the uniquely strong collagen that allows mussels to stick so aggressively
to everything from rocks to oil-rigs. The finding could lead to new bionic
materials, along with new techniques for removing the pesky mollusks from
manmade surfaces.
Mussels produce fibers called byssal threads which are five times
tougher and 16 times more extensible than a human tendon. The newly discovered
protein is believed to be the first known protein with both collagenous
and elastin-like domains. Byssal threads feature "a stiff tether" at one
end and a "shock absorber" on the other end protruding from the mussel
foot.
"Mussels create a major fouling problem on economically important surfaces
exposed to the sea," says J. Herbert Waite, professor of marine biochemistry,
University of Delaware. "If a byssal thread were simply a stiff stick attached
to an elastic tube, it wouldn't have an outside chance of surviving these
relentless tidal beatings. In fact, the collagen in byssal threads goes
from being a stiff material, to something that's very stretchy--without
any sudden transitions."
The researchers isolated two key collagens: Col-P and Col-D, from the
mussels. They used pepsin, an enzyme secreted by stomach cells of vertebrates,
to pinpoint the collagens. Unlike most proteins, Col-P and Col-D don't
break apart in response to pepsin. Since pepsin works best in an acidic
environment, researchers simply placed byssal threads and pepsin in a weak
solution of acetic acid.
"Col-P and Col-D were the only proteins detectable after pepsinization,"
Waite explains. "Within byssal threads, these two collagens are distributed
in a complementary gradient, with Col-P predominant in the elastic, proximal
region, near the foot of the mussel, and more Col-D at the far, or distal
end."
The investigators also examined the protein precursors for Col-P and
Col-D, found in the mussel foot, where byssal threads are produced. The
then used antibodies to target preCol-P. First, messenger RNA (ribonucleic
acid) containing the genetic code for the protein was extracted from mussel
foot tissue. Next, RNA was converted to the more stable DNA (deoxyribonucleic
acid) form and cloned into bacteria, which expressed the protein encoded
by the mussel's RNA. Designer antibodies, produced by a laboratory animal
injected with Col-P, quickly latched onto the protein precursor expressed
by bacterial clones.
The researchers found that preCol-P contained three major domains- a
tough collagen-based domain, flanked on either side by a pair of elastin-like
regions. These stretchy domains are framed at each end by sections rich
in the amino acid, histidine.
The rubbery sections of preCol-P resemble bovine elastin, which is "very
similar" to human elastin, noted lead investigator Kathryn J. Coyne.
"Elastins typically are found in the skin and arteries of vertebrate species
only," she notes. "The presence of these types of sequences in proteins
from an invertebrate species is unusual."
The elastin-like regions of preCol-P also contained high levels of glycine
and alanine--the amino acids most prevalent in two forms of protein in
spider silk, Waite says. Although the structural similarity between preCol-P
and spider silk still must be verified, Waite says the possibility should
interest biochemists. "Spider silk is so thin, it has been difficult for
anyone but crystallographers to deal with it," he says. "Byssal threads
could turn out to be an interesting substitute, or model for studying some
aspects of spider silk."
Curiously, the collagenous regions of preCol-P contain a missing glycine.
"When a deletion like this is found in other structural collagens," Waite
says, "it's certainly lethal to the animal. So, it's quite fascinating
to find a missing glycine in a perfectly functional collagen subjected
to great stress and strain in marine environments." It's possible, Waite
speculates, that the missing glycine creates a 35-degree "kink" or bend
in the collagen. But, he adds, "how that might contribute to the stretchiness
of the protein is anybody's guess."
The researchers also believe two histidine-rich domains, located at
each end of preCol-P, may play a role in forming protein-zinc complexes.
Whenever histidine-rich domains occur in proteins, they usually bind with
metal. In blood, for instance, histidine-loaded glycoprotein binds with
zinc. In byssal threads these domains may react with metals to produce
strong "bridges," which link up linear arrays of collagen. Breaks in some
of these cross-linked sections appear to be promptly repaired, Waite says.
"This is totally different from how vertebrate collagens form fibers
in nature," he notes. "Bridge formation seems to be reversible, so that
if you pull them apart, they reform when you bring them back into contact."
It's not yet feasible to manufacture materials featuring such gradual
transitions, Waite says. But, he adds, "It's fun to dream about versatile
new materials for a whole host of products--from steel-belted radials to
shoes, which must be soft and flexible, yet tough enough to pound the pavement."
An improved understanding of byssal threads might help scientists design
biomaterials that take advantage of their remarkable properties, says Harold
Slavkin, director of the National Institute of Dental Research, one of
the National Institutes of Health, which sponsored the study. "Insight
into the molecular structure that makes the byssus strong yet flexible
might suggest, for example, new strategies for designing more comfortable
and pliable artificial skin."
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