ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
KL16. Towards the Structure and Properties of Plant Metallothioneins
E. Freisinger
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland.
As their vertebrate relatives, plant metallothioneins (MTs) are small cysteine-rich proteins with a preference for
metal ions with the electron configuration d 10 . They have been proposed to play a role in the homeostasis of
metal ions such as Zn II and Cu I . Some plant MTs also seem to function in the detoxification of heavy metals like
e.g. Cd II , most likely in combination with enzymatically synthesized cysteine-rich peptides called phytochelatins.
Gene expression studies further show inducibility of mt gene expression in response to stress conditions such as
draught, leave wounding, and scenescence.
In the past, research activity on the actual gene products, the plant MT proteins themselves, has been low only
increasing in the last few years. Accordingly, informations about the structures and functions of these proteins
are scarce, in particular, no three-dimensional structure is available from the literature so far. This is even more
surprising considering the amino acid sequence diversity plant MTs display, especially in comparison to the
vertebrate isoforms. This diversity has let to a further differentiation of the plant MTs into four subfamilies
mainly based on the respective cysteine-distribution pattern. In part, the low research activity might be due to
difficulties in isolating enough protein material to perform experiments. However, with cloning and recombinant
expression techniques becoming more and more common including the use of fusion proteins for the expression
of small proteins and peptides, the production of highly pure and homogeneous protein material is no longer the
major bottleneck.
Current knowledge about plant MTs includes the
number of divalent metal ions they are able to
coordinate in form of metal-thiolate clusters, the
pH stability of the Zn II - and Cd II -thiolate clusters
based on the apparent pKa values of the cysteine
thiolate groups in presence of the respective metal
ions, the presence of secondary structural elements,
namely �-sheets, in the long cysteine-free linker
regions connecting cysteine-rich regions, as well as
the number and arrangement of metal-thiolate
clusters in selected proteins [1�3]. This contribution
will summarize the present research status and
allow new insights into the structures and metal ion
coordination abilities of plant MTs based on
spectroscopic methods such as UV-Vis, circular
dichroism, and NMR spectroscopy, dynamic light
scattering experiments and limited proteolytic
digestion reactions to probe the number of metalthiolate
clusters formed and their arrangement
along the amino acid chain. In addition, metal ion
substitution and reconstitution reactions followed
by mass spectrometry provide exciting informations about the cluster formation processes and reveal metal ion
coordination sites with reduced as well as increased stabilities.
Acknowledgement: Financial support from the Swiss National Science Foundation is gratefully acknowledged:
SNF grant 20-113728/1 and SNF-Förderungsprofessur PP002-119106/1.
References:
[1] E. Freisinger, Inorg. Chim. Acta, 360, 369 (2007).
[2] E. A. Peroza, E. Freisinger, J. Biol. Inorg. Chem., 12, 377 (2007).
[3] O. Schicht, E. Freisinger, Inorg. Chim. Acta, doi: 10.1016/J.ica.2008.03.097 (2008).
_____________________________________________________________________
35