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Re: Help Please





I also vaguely remember another more recent post about a presentation in which the differences in protein were covered in more detail. There may have been a news article about the presentation. Still searching. Hope this is useful.

Doing a bit of digging in my own archives, I find I posted this as a summary of Mary Schweitzer's paper at the Ostrom Symposium:


"The feathers of birds are made only of beta keratin, a protein unknown in mammals. The beta keratin in feathers is further distinguished from other beta keratins by a deletion.

"Schweitzer has found that immunological tests on feathers of modern birds express only beta keratin. When she tried the same test on the fibres associated with Shuvuia, she got the same result (as I am not really knowledgeable about the procedure, I am not sure if she was able to determine whether the beta keratin involved was the special type found in bird feathers - can anyone enlighten me on this?)."

To which Alan Brush responded:

"Actually this is much more complex that it seems. Schwitzer presented a slide that illustrates feather keratin that was based on a diagram in my article in Avian Biology (vol 9, 1993). This point is that the family of feather keratins genes (which produced the main structural proteins in ALL kinds of feathers, beaks, scale and scutes)has undergone some evolution since their origin. The original gene was duplicated several times. Subsequently some members underwent a deletion. So the family contains 2 sets of genes, each of which is duplicated several times and exists in tandem repeats in the genome. The evidence for this comes from sequences work on both the proteins and the genes. We know precisely where the deletion occurred. Both the beta-pleated sheet and the Cys containing proteins of the proteins are conserved. This is functionally important. Anyway, the expresses genes exist in tandem arrays of 15-18 members. These can differ in AA sequence. The entire sets (one for contour feathers, another for down, another for scale, etc) are probably repeats 125-150 times throughout the genome. This arrangement, which is different from other epidermal appendages, is highly conserved and determines properties of the structures such as the nature of the filaments and structural molecular organization of the feathers themselves.

"It does bring up a problem, one that was mentioned in the discussion. That is in order to use anti-sera to identify feathers, you need to have very pure antigens. With this relatively large number, it is not an easy task!

"It is also important to consider from an evolutionary viewpoint that with the feather keratin genes excepted, the genes that determine the linear nature, ramification, growth and timing of feather exist widely in animals. The regulatory events that lead to feather productions are probably a consequence of common cell functions and cytodifferentation. What emerged from the combination of the feather keratins and follicle was a extremely adaptable system capable of producing great phenotypic variation. All excellent conditions of rapid evolutionary change."

Ralph Miller on the Schweitzer paper:

"CLAWS, BEAKS, SCALES AND FEATHERS: THE EVOLUTIONARY IMPLICATIONS OF KERATIN
PRESERVATION IN THE FOSSIL RECORD
Mary Higby Schweitzer, Dept. of Biology, Museum of the Rockies, Montana State
University, Bozeman, MT 59717
Keratin is the structural protein responsible for making skin waterproof and
durable, and it is the key component of hair, nails, claws, and other epidermal
structures. The durability and "hardness" of keratinous elements is a result of
the molecular composition and structure of these proteins.
The keratins are a phylogenetically significant family of proteins. Alpha keratin
arose from alteration and subsequent divergence in a protein which constitutes the
cytoskeletal system of epidermal cells. This alteration arose with the
vertebrates, and the cornified skin layers of all vertebrates contain alpha
keratin. Beta keratin, on the other hand, is a family of proteins which arose
from an unidentified cellular precursor, sometime after the divergence of mammals,
and so is unique to reptiles and birds among extant taxa.
Traces of keratinous structures are recorded often in the fossil record. The most
well known of these are feathers, which are found in sediments dating to the
Jurassic, and which are used to identify the geographical and temporal range of
birds.
Although structures originally composed of keratin are not exceedingly rare in the
fossil record, it has been assumed that none of the original molecular components
persist, and that the structures are mineral replications. Recent studies,
however, support the hypothesis that some remnants of the original protein
structures remain in at least some specimens, preserved in unique depositional
settings.
Antibodies specific to keratin proteins were applied to two Cretaceous specimens.
Fibrous material adhering to the ungual of a bird from Madagascar reacted
positively and strongly to both alpha and beta keratin antibodies, and did not
react to non-specific antibodies present in normal sera, or to antibodies raised
against a non-relevant protein. This supports the conclusion that both alpha and
beta keratins were present in this material, as is the case for keratinous claw
sheaths for modern birds.
A specimen of _Mononykus_ was recovered from the Gobi desert, and small white
fibers were noted in the sediments surrounding the skeleton. The fibers were
localized to the skeleton, and arrayed in a manner suggestive of feathers.
Microscopic and chemical analyses eliminated plant material or fungal hyphae as a
source for these fibers. Antibodies specific for beta keratin reacted strongly
with these fibers, while antibodies against alpha keratin, antibodies present in
normal sera, and antibodies raised against a non-relevant protein were negative
for binding, a pattern consistent with modern feathers.
These two specimens demonstrate that useful molecular information can be recovered
from the fossil record. In addition, these data may be useful in shedding light
on evolutionary processes within the bird/dinosaur lineage."


Toby White posted the following on the same subject:

"Keratin is one of the most common proteins on earth. Skin is rich in
keratin and leather is, in fact, almost pure keratin. Looking for traces of
keratin residues invites artifacts because the stuff could have come from
anywhere, including the skin of a preparator or the scales or skin of a
completely featherless dinosaur. In fact, keratin is a major structural
component of hair, wool, skin, scales, nails, hooves and horns as well as
feathers.
<omitted daydream about a flying Jurassic sheep>
Schweitzer et al. note that what they saw was beta keratin. The difference
between alpha and beta keratin is largely conformational, not chemical. Are
they really maintaining that the protein, not only survived, but maintained
its three-dimensional structure for a hundred million years?
Their second point is that only beta keratin was found. Assuming this is
not a contaminant (a major assumption), it is not at all clear that the
conditions under which the fossil was preserved would permit alpha keratin
to exist in that form. Moist heat, in particular can break up the helical
conformation of alpha keratin. In any case, if their premise is that beta
keratin is only found in feathers, they're simply wrong.
Third, they apparently stated that only beta keratin was found. That is,
that no other protein species was recovered in measurable amounts. This is
unsurprising whether or not the keratin was a contaminant. Again assuming
no contamination, keratin is very common, very insoluble, and individual
keratin fibres are very sticky with respect to each other. Thus it is hard
to digest and will be preferentially preserved when everything else is
either dissolved or degraded. The fact that nothing else was recovered may
only mean that (a) it was very abundant to begin with and (b) everything
else got eaten up or washed away (either naturally or by the prep method).
Finally, the tubes. Keratin molecules are sticky in the beta conformation
because adjacent strands hydrogen bond to each other. This conformation,
the "beta sheet," is energeticly favored strongly whether the strands are
parallel or antiparallel. If the number of strands falls within a fairly
broad range of sizes, there should be a strong energetic reason for the
strands to curl around to form a tube-like structure. In this conformation,
all of the hydrogen bonding is with other keratin strands rather than the
less-favored interaction with water. (The SEM prep method could affect the
result as well.)
I'd be real careful about citing this one. Its an interesting result, and
may be correct, but there's an awful lot that could have gone wrong."

Also, Mickey Mortimer noted the following paper:

Sawyer, Glenn, French, Mays, Shames, Barnes, Rhodes and Ishikawa, 2000. The expression of Beta keratins in the epidermal appendages of reptiles and birds. American Zoologist 40 (4): 530-539.




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Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
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