Structural features specific to plant metallothioneins |
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Authors: | Eva Freisinger |
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Institution: | (1) Institute of Inorganic Chemistry, University of Zurich, 8057 Zurich, Switzerland |
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Abstract: | The metallothionein (MT) superfamily combines a large variety of small cysteine-rich proteins from nearly all phyla of life
that have the ability to coordinate various transition metal ions, including ZnII, CdII, and CuI. The members of the plant MT family are characterized by great sequence diversity, requiring further subdivision into four
subfamilies. Very peculiar and not well understood is the presence of rather long cysteine-free amino acid linkers between
the cysteine-rich regions. In light of the distinct differences in sequence to MTs from other families, it seems obvious to
assume that these differences will also be manifested on the structural level. This was already impressively demonstrated
with the elucidation of the three-dimensional structure of the wheat Ec-1 MT, which revealed two metal cluster arrangements previously unprecedented for any MT. However, as this structure is so
far the only one available for the plant MT family, other sources of information are in high demand. In this review the focus
is thus set on any structural features known, deduced, or assumed for the plant MT proteins. This includes the determination
of secondary structural elements by circular dichroism, IR, and Raman spectroscopy, the analysis of the influence of the long
linker regions, and the evaluation of the spatial arrangement of the sequence separated cysteine-rich regions with the aid
of, e.g., limited proteolytic digestion. In addition, special attention is paid to the contents of divalent metal ions as
the metal ion to cysteine ratios are important for predicting and understanding possible metal–thiolate cluster structures. |
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