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Published 7 November 2005. doi:10.1083/jcb.200506171
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 171, Number 3, 425-430
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Complete maturation of the plastid protein translocation channel requires a type I signal peptidase



Kentaro Inoue1, Amy J. Baldwin1, Rebecca L. Shipman1, Kyoko Matsui3, Steven M. Theg2, and Masaru Ohme-Takagi3

1 Department of Plant Sciences, College of Agricultural and Environmental Sciences
2 Section of Plant Biology, College of Biological Sciences, University of California, Davis, Davis, CA 95616
3 Gene Function Research Laboratory, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8566, Japan

Correspondence to Kentaro Inoue: kinoue{at}ucdavis.edu


Abstract

The protein translocation channel at the plastid outer envelope membrane, Toc75, is essential for the viability of plants from the embryonic stage. It is encoded in the nucleus and is synthesized with a bipartite transit peptide that is cleaved during maturation. Despite its important function, the molecular mechanism and the biological significance of the full maturation of Toc75 remain unclear. In this study, we show that a type I signal peptidase (SPase I) is responsible for this process. First, we demonstrate that a bacterial SPase I converted Toc75 precursor to its mature form in vitro. Next, we show that disruption of a gene encoding plastidic SPase I (Plsp1) resulted in the accumulation of immature forms of Toc75, severe reduction of plastid internal membrane development, and a seedling lethal phenotype. These phenotypes were rescued by the overexpression of Plsp1 complementary DNA. Plsp1 appeared to be targeted both to the envelope and to the thylakoidal membranes; thus, it may have multiple functions.

A.J. Baldwin and R.L. Shipman contributed equally to this paper.

Abbreviations used in this paper: Lep, leader peptidase; OE33, 33-kD component of the oxygen-evolving complex; PLB, prolamellar body; Plsp1, plastidic SPase I; POR, protochlorophyllide oxidoreductase; SPase I, type I signal peptidase; Toc, translocon at the outer envelope membrane of chloroplasts.


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