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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

A Pentapeptide Motif Related to a Pigment Binding Site in the Major Light-Harvesting Protein of Photosystem II, LHCII, Governs Substrate-Dependent Plastid Import of NADPH:Protochlorophyllide Oxidoreductase A^1,2,[W]

Universität Bayreuth, Lehrstuhl für Pflanzenphysiologie, D–95447 Bayreuth, Germany (C.R.); Ruhr-Universität Bochum, Lehrstuhl für Pflanzenphysiologie, D–44801 Bochum, Germany (S.P., S.R.); Université Joseph Fourier et Centre National de la Recherche Scientifique, F–38041 Grenoble cedex 9, France (P.P.-F., F.Q., S.R.); and Department of Molecular Cell Biology, Utrecht University, 3584 Utrecht, The Netherlands (P.W.)

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) A is the only known example thus far of a nucleus-encoded plastid protein that is imported to its final destination in a substrate-dependent, Pchlide-regulated manner. Previous work has shown that the cytosolic PORA precursor (pPORA) does not utilize the general import site but uses a distinct translocon designated the Pchlide-dependent translocon complex. Here we demonstrate that a pentapeptide motif, threonine-threonine-serine-proline-glycine (TTSPG) in pPORA's transit peptide (transA), is involved in Pchlide-dependent transport. Deletion of this motif from the COOH-terminal end of transA abolished both Pchlide binding and protein import. Incorporation of the TTSPG motif into normally non-Pchlide-responsive transit sequences conferred the pigment binding properties onto the engineered chimeric precursors but was insufficient to render protein import substrate dependent. An additional motif was identified in the NH₂-terminal part of transA that was needed for binding of the precursor to the Pchlide-dependent translocon complex. Point mutations of the TTSPG motif identified glycine as the Pchlide binding site. By analogy to the major light-harvesting chlorophyll a/b binding protein of photosystem II, we propose that the peptidyl carbonyl oxygen of glycine may bind directly or via a water molecule to the central Mg atom of the pigment.

² This article is dedicated to Klaus Apel on the occasion of his 65th birthday.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Christiane Reinbothe (christiane.reinbothe{at}uni-bayreuth.de).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.120113

^* Corresponding author; e-mail christiane.reinbothe{at}uni-bayreuth.de.

Received April 3, 2008; accepted April 15, 2008; published April 25, 2008.