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J. Biol. Chem., Vol. 281, Issue 3, 1449-1460, January 20, 2006

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Glutamic Acid-rich Proteins of Rod Photoreceptors Are Natively Unfolded^*

Renu Batra-Safferling, Karin Abarca-Heidemann, Heinz Gerd Körschen, Christos Tziatzios¶, Matthias Stoldt||**, Ivan Budyak||, Dieter Willbold||**, Harald Schwalbe, Judith Klein-Seetharaman||, and U. Benjamin Kaupp1

From the Institut für Biologische Informationsverarbeitung 1 and the ^||Institut für Biologische Informationsverarbeitung 2, Forschungszentrum Jülich, 52425 Jülich, ^**Institut für Physikalische Biologie, Heinrich-Heine-Universität, 40225 Düsseldorf, Zentrum für Biologische Magnetische Resonanz/Institut für Organische Chemie und Biochemie, J. W. Goethe-Universität Frankfurt, Marie-Curie-Strasse 11, 60439 Frankfurt, and ^¶Institut für Biophysik, J. W. Goethe-Universität, 60590 Frankfurt, Germany

The outer segment of vertebrate photoreceptors is a specialized compartment that hosts all the signaling components required for visual transduction. Specific to rod photoreceptors is an unusual set of three glutamic acid-rich proteins (GARPs) as follows: two soluble forms, GARP1 and GARP2, and the N-terminal cytoplasmic domain (GARP' part) of the B1 subunit of the cyclic GMP-gated channel. GARPs have been shown to interact with proteins at the rim of the disc membrane. Here we characterized native GARP1 and GARP2 purified from bovine rod photoreceptors. Amino acid sequence analysis of GARPs revealed structural features typical of "natively unfolded" proteins. By using biophysical techniques, including size-exclusion chromatography, dynamic light scattering, NMR spectroscopy, and circular dichroism, we showed that GARPs indeed exhibit a large degree of intrinsic disorder. Analytical ultracentrifugation and chemical cross-linking showed that GARPs exist in a monomer/multimer equilibrium. The results suggested that the function of GARP proteins is linked to their structural disorder. They may provide flexible spacers or linkers tethering the cyclic GMP-gated channel in the plasma membrane to peripherin at the disc rim to produce a stack of rings of these protein complexes along the long axis of the outer segment. GARP proteins could then provide the environment needed for protein interactions in the rim region of discs.

Received for publication, May 6, 2005 , and in revised form, September 7, 2005.

^* This work was supported by the Schwerpunktprogramm of the Deutsche Forschungsgemeinschaft "Molekulare Sinnesphysiologie" SPP 1025 and the Sofya Kovalevskaya award (Humboldt-Foundation and Zukunftsinvestitionsprogramm der Bundesregierung Deutschland). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains a table and Figs. 1 and 2.

¹ To whom correspondence should be addressed: Institut für Biologische Informationsverarbeitung 1, FZJ-IBI-1, 52425 Jülich, Germany. Tel.: 492461-614041; Fax: 492461-614216; E-mail: a.eckert{at}fz-juelich.de.

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