From: ncbi-seminar-admin@ncbi.nlm.nih.gov on behalf of Eugene Koonin
[koonin@golem.nlm.nih.gov]
Sent: Wednesday, September 11, 2002 8:06 PM
To: Anurag K Mishra; ncbi-seminar@golem.nlm.nih.gov
Subject: Re: A postdoctoral application

Bldg. 38A, 5th floor Conference room
Monday 09.16, 3 PM



Crocodilian crystallins: an evolutionary perspective

Anurag Mishra
Center for Cellular and Molecular Biology
Hyderabad, INdia


Eye lens proteins crystallins have been interesting models not only to
study the structure and ageing, but also to look into the phenomenon of
gene sharing and evolution since they are thought to have evolved by the
process of gene duplication and divergence. Lens crystallins can be
classified into two groups; the ubiquitous alpha, beta and
gamma-crystallins present in all vertebrate lenses, and the taxon-specific
crystallins, restricted to only certain taxa. Taxon-specific crystallins
are the only known structural proteins, which are often recruited in
non-lenticular tissues to perform an altogether different function.

Crocodiles, being the living fossils provide an excellent system for
studying the evolutionary aspect of gene sharing and protein properties.
With our investigations into the crocodilian lens, it was found to be
having at least six different types of crystallins, thus providing a unique
kind of lens; so far no other species has been identified with these many
lens crystallins. The identification and characterization of individual
crystallin components indicated that crocodilian lens was by no means a
simple tissue. As many of the taxon-limited crystallins are not found in
higher mammals, it became all the more interesting to probe as to why these
crystallins were recruited in their eye and the possible reasons behind
their extinction in later stages of evolution. Crocodilian lens crystallin
components turned out to be interesting due to many undiscovered startling
observations:

Evolution of a protein property: alpha-crystallin, one of the principal
lens constituents, is a well- known chaperone. The most startling
observation was the highly diminished chaperone activity in crocodilian
alpha-crystallin. This is the first observation of its kind, which provides
the unique example of chaperone-less alpha-crystallin, besides indicating
as to how chaperone-like activity might have evolved. We suggest that
alpha-crystallin had negligible or nil chaperone activity in the earlier
stages of evolution, which evolved slowly, catering to the growing need of
eye lenses for adjustment under stressful situations. The results not only
signify the structural requirement of the protein to be a chaperone, but
also point towards the evolution of this important function.

Perfect gene duplication: tau-crystallin, which is a major crystallin in
reptiles such as turtle, is a very minor component of crocodilian lenses.
It is also present in some aquatic birds. The lowest level of
tau-crystallin in the crocodilian lenses attracted us to look at the
features of this protein. We looked upon the gene sequence of
tau-crystallin, and found it to be identical to alpha-enolase from other
non-lenticular tissues (brain, heart and gonad). The existence of the same
gene product in the lens as well as in non-lenticular tissue established
the dual function of this protein. Though the phenomena of gene sharing
have been described earlier, ours is an additional proof to show the dual
expression of tau-crystallin and alpha-enolase. In addition, we have found
that tau-crystallin is a monomeric protein, whereas alpha-enolases are
known to be dimeric. The study also throws up a number of interesting
queries to be pursued as future line of research like; what are the reasons
behind the low level expression of tau-crystallin in crocodiles and its
subsequent loss during evolution? And an interesting possibility, since the
protein seems to be prevalent in water dwelling reptiles and birds, is it
present in water-dwelling mammals as well?

Novel crystallins in crocodilian lenses: In addition, we have found at
least two lens crystallins in crocodiles, which have not been reported so
far. Preliminary studies have been done to characterize these novel
crystallins in crocodilian eyes. It will be interesting to determine their
function, and to discover if they too are recruited for a possible dual
role? The results suggest that there could be more genes recruited for dual
functions.





--
Eugene V. Koonin, PhD
National Center for Biotechnology Information
National Library of Medicine
National Institutes of Health, Bldg. 38A, Rm. 5N503 (5th floor) 8600
Rockville Pike Bethesda, MD 20894, USA
Phone: (301)435-5913; Fax: (301)435-7794 or (301)480-9241
email koonin@ncbi.nlm.nih.gov
http://www.ncbi.nlm.nih.gov/CBBresearch/Koonin/