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Metallothioneins of the South American opossum, Monodelphis domestica

Miller, Janine Margaret (1998) Metallothioneins of the South American opossum, Monodelphis domestica. PhD thesis, University of Tasmania.

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Abstract

Metallothioneins are small, cysteine-rich, metal-binding proteins which
occur ubiquitously throughout the animal kingdom. After several decades of
research, a definitive function for these metallothioneins has yet to be clarified.
Many metallothioneins have been characterised in the placental mammals (subclass
Eutheria) and their occurrence in complex multigene families has been confirmed.
Subtle differences in protein structure and expression exist between metallothionein
isoforms but it has yet to be established whether the various isoforms play differing
roles physiologically. Of the vertebrates, the mammals alone manifest complex MT
gene families. The birds, by contrast, express one or rarely two metallothionein
genes.
The marsupials represent a mammalian subclass (Metatheria) distinct from
the placental mammals. Gross differences in reproductive biology distinguish the
two groups, but in other respects the marsupials differ only slightly from placental
mammals in fundamental physiology. From several perspectives, characterisation of
a marsupial MT should extend our understanding of the physiological role and the
evolution of the mammalian metallothioneins. Marsupial metallothionein is
identified here for the first time.
Several independent marsupial MT sequences were isolated from both liver
and brain-derived cDNA libraries. The predicted marsupial MT proteins do not
conform closely to eutherian isoforms when the traits of cognate proteins are
compared, and cannot therefore be accommodated within the current classification
system. The tissue-specific expression of the marsupial metallothioneins was
investigated to further clarify their relationship to the eutherian metallothioneins.
Here too, no confident correlation is possible. The specific and predictable
expression profile of MT in the perinatal eutherian was compared with that of the
marsupial neonate, which is relatively and strikingly underdeveloped. Marked
differences in these profiles were observed.
Like the eutherian mammals, the marsupial mammals are here shown to
express a complex family of metallothionein genes. Phylogenetic analysis of the vertebrate metallothionein gene family indicates that the amplification of ancestral
genes into multiple isoforms occurred independently in both mammalian lineages. It
can be inferred that the multiplicity of MT isoforms has arisen not just in response
to the physiological requirements that distinguish the mammals, but to some
stimulus that acted upon their particular physiology a considerable time after its
establishment. Two metallothionein cDNA sequences from the echidna, an
Australian monotreme of the mammalian subclass Prototheria, are included in the
analysis and confirm these conclusions. It is shown that the mammalian
metallothioneins have evolved from an MT3 homologue closely related to bird MT.
The eutherian MT1 and MT2 isoforms arose subsequent to the separation of the
eutherian and metatherian lineages, approx. 120 myrs ago. The position of the MT4
isoform is less clear but is likely to represent the most ancient of the metallothionein
proteins.

Item Type: Thesis (PhD)
Keywords: Opossums, Metallothionein
Copyright Holders: The Author
Copyright Information:

Copyright 1998 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1999. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:03
Last Modified: 13 Nov 2017 23:23
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