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Thomas Smith's Laboratory |
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The fut operon
Both Gram-negative and Gram-positive bacteria have evolved ATP binding cassette-type transport systems for the
high affinity uptake of transition metal ions such as iron, manganese, and zinc, particularly at low extracellular levels of these metals. In Synechocystis 6803, the futA1 (slr1295) and futA2 (slr0513) genes encode periplasmic-binding proteins that have been proposed to be part of a high affinity iron transport system. Previously, in vitro studies with purified FutA1 expressed as a GST2 fusion protein in Escherichia coli suggested that the protein binds ferric iron with an association constant of about 0.1 picomolar. Similarly, in Synechocystis 6803 mutants in which futA1 and futA2 were deleted, iron uptake was reduced to 37 and 84%, respectively, compared with wild type. When both genes were deleted, the resulting iron uptake activity was less than 5% that of wild type. To understand the specificity and iron recognition properties of FutA1, we determined its structure and performed binding analysis.
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Atomic structure of FutA1
Comparison of the structures of FutA1 and cFbpA from C. jejuni. In this image the rainbow-colored ribbon diagram represents FutA1 colored from blue to red as the protein extends from the amino to carboxyl termini. The view is looking into the iron-binding site. The transparent gray ribbon figure is that of cFbpA that was overlaid onto FutA1.
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Iron binding environment
The iron-binding environment in FutA1. The left figure shows the electron density of iron bound to FutA1 and some of the contact residues, and the right figure shows some of the secondary shell contacts with the ligating residues. The orientation here is similar to that in Fig. 1 with the N-terminal domain toward the bottom of the figure and looking into the iron binding pocket. |
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Iron binding environment
Changes in the iron-binding site upon iron release. In this figure the ligating residues in the iron bound FutA1 structure are represented by the transparent stick figures, and the apo form is represented by the solid structure.
Unlike ZnuA, the ligating residues appear to be relatively fixed within the two domains and the domains move a great deal upon binding and release of the iron. The residues on the top belong to one domain and the residues on the bottom belong to the second domain. In ZnuA, the two domains remain fixed and the residues binding the metal rotate into and out of the binding environment. |
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Changes in the FutA1 structure upon iron binding
Changes in the contact area between the two domains as the iron binding cleft opens and closes. In this figure the N-terminal domain is represented by the surface rendering, and the C-terminal domain is represented by the transparent green ribbon diagram. The overall structure used for this figure is of the apo form. The red-colored surface is that unique part of the N-terminal/C-terminal domain interface that is buried upon opening of the iron-binding cleft, whereas the blue portion is buried upon the closing of the iron binding cleft.
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Our relevant publications:
- Koropatkin, N. M., Randich, A. M., Bhattacharyya-Pakrasi, M., Pakrasi, H. B., Smith, T. J. (2007) The structure of the iron binding protein, FutA1, from Synechocystis 6803. J. Biol. Chem. 282:27468-27477.
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