Poster: Organelle Biogenesis

Abs # 1140: csr1, the maize mutants of chloroplast FtsY exhibit a pleiotrophic defect in biogenesis of thylakoids

Presenter:	Asakura, Yukari , han@protein.osaka-u.ac.jp
Authors	Asakura, Yukari  (A)   Hirohashi, Toshiya  (A)   Kikuchi, Shingo  (A)   Belcher, Susan  (B)   Osborne, Erin  (B)   Barkan, Alice  (B)   Yano, Satoshi  (C)   Terashima, Ichiro  (C)   Nakai, Masato  (A)
Affiliations:	(A): Institute for Protein Research, Osaka University (B): Institute of Moleculer Biology, University of Oregon (C): Depertment of Biology, Graduate School of Science, Osaka University

The chloroplast signal recognition particle (SRP) pathway, which is used for the insertion of light-harvesting chlorophyll proteins (LHCPs) into the thylakoid membranes, involves cpSRP43 and cpSRP54 as soluble components, an SRP receptor cpFtsY on the membrane, and Alb3 as an integral membrane component. Although physiological significances of the roles of cpSRP43, cpSRP54, and Alb3 on thylakoid biogenesis have been well studied using their respective mutant plants, that of cpFtsY remains to be investigated. Here we characterized maize cpFtsY and its relevant csr (chloroplast SRP receptor) mutants having a Mu transposon insertion in the corresponding gene. Maize cpFtsY was expressed mainly in leaves but not in roots and was also present in etiolated leaves. In chloroplasts, cpFtsY was found to exclusively bind on the thylakoids and, in etioplasts, on the prolameller bodies. A null cpFtsY mutant csr1-1 showed a pale yellow-green phenotype and was seedling lethal. The csr1-1 chloroplasts showed remarkably unstacked and reduced thylakoid membranes, while prolameller bodies in etioplasts seemed to be normal. In vitro import study revealed that the integration of LHCP into the thylakoid membranes of the mutant was indeed impaired. In addition to reduced levels of various LHCP proteins, the mutant exhibited severe pleiotropic defects in the assembly of other thylakoid proteins whose thylakoid targeting has been known to use Sec or Tat pathways, despite the fact that known components of Sec and Tat machineries themselves were not severely affected. Our results demonstrate the essentiality of cpFtsY for overall organization of photosynthetic components during thylakoid development coordinated with efficient assembly of abundant LHCPs into the thylakoid membrane.