better beta keratin

[Bill Adlam <sagitta_elegans@yahoo.com>]

Laurie Nyveen wrote:

> >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.
> 
> OK, I'm confused.  Keratin is keratin - it has one molecular formula
(more
> or less - there are about 30 known varieties).  There's two structural
> forms of it - a helix (called alpha-keratin) and a pleated sheet
(called
> beta keratin).
> 
> Now, if this is what she's talking about, then, yes, all vertebrates
have
> alpha-helix keratin (I haven't been able to confirm in the short
time I
> looked that inverts don't). However, beta keratin did NOT arise after
> mammals split off, since inverts produce it.  Silk and spiderweb, in
fact,
> are made of beta keratin.

I'm not surprised you're confused.  Yes, keratin is keratin.  Alpha
keratin definitely has an alpha-helix secondary structure.  I don't
know about the beta keratin in feathers.

Unfortunately, a lot of molecular biologists use the term beta keratin
to refer to various fibrous proteins with a beta pleated sheet
structure.  Like fibroin, the moth silk protein.  As far as I know,
these 'beta keratins' aren't related to vertebrate keratins.  I don't
know if they're even related to each other.

I hope this makes things less confusing.

Bill Adlam
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