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Canonical Signal Recognition Particle Components Can Be Bypassed for Posttranslational Protein Targeting in Chloroplasts^[W]

^a Unité Mixte de Recherche 6191, Centre National de la Recherche Scientifique–Commissariat à l'Energie Atomique-Universite Aix-Marseille II, Direction des Sciences du Vivant, Institut de Biologie Environnementale et de Biotechnologie, Service de Biologie Végétale et de Microbiologie Environnementale, Commissariat à l'Energie Atomique Cadarache, F-13108 Saint-Paul-lez-Durance Cedex, France
^b Biological Sciences Department, University of Arkansas, Fayetteville, Arkansas 72701
^c Unité Mixte de Recherche 619, Biologie du Fruit, Plateforme Imagerie Institut Fédératif de Recherche 103, Institut de Biologie Végétale Moléculaire, Institut National de la Recherche Agronomique Centre de Bordeaux, 33883 Villenave d'Ornon Cedex, France
^d Biochemie-Zentrum der Universitat Heidelberg, D-69120 Heidelberg, Germany
^e Direction des Sciences du Vivant, Institut de Biologie Environnementale et de Biotechnologie, Service de Biochimie Post-Génomique et Toxicologie Nucléaire, Commissariat à l'Energie Atomique-Valrho-BP 17171, 30207 Bagnols/Cèze Cedex, France
^f Environmental Risk Analysis Division, Biotechnology Regulatory Services, Animal and Plant Health Inspection Service/U.S. Department of Agriculture, Riverdale, Maryland 20737

¹ To whom correspondence should be addressed. E-mail lnussaume{at}cea.fr; fax 33-442-254597.

The chloroplast signal recognition particle (cpSRP) and its receptor (cpFtsY) target proteins both cotranslationally and posttranslationally to the thylakoids. This dual function enables cpSRP to utilize its posttranslational activities for targeting a family of nucleus-encoded light-harvesting chlorophyll binding proteins (LHCPs), the most abundant membrane proteins in plants. Previous in vitro experiments indicated an absolute requirement for all cpSRP pathway soluble components. In agreement, a cpFtsY mutant in Arabidopsis thaliana exhibits a severe chlorotic phenotype resulting from a massive loss of LHCPs. Surprisingly, a double mutant, cpftsy cpsrp54, recovers to a great extent from the chlorotic cpftsy phenotype. This establishes that in plants, a new alternative pathway exists that can bypass cpSRP posttranslational targeting activities. Using a mutant form of cpSRP43 that is unable to assemble with cpSRP54, we complemented the cpSRP43-deficient mutant and found that this subunit is required for the alternative pathway. Along with the ability of cpSRP43 alone to bind the ALBINO3 translocase required for LHCP integration, our results indicate that cpSRP43 has developed features to function independently of cpSRP54/cpFtsY in targeting LHCPs to the thylakoid membranes.

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