PARENT SESSION

Symposium S7C Biosynthesis and assembly: Protein trafficking
Thursday September 2nd, 2004 2:40 PM-4:40 PM Room 510B
Chair: Ken Cline
Co-Chair: Steven Theg

Involvement of a type I signal peptidase in biogenesis of chloroplasts. Kentaro Inoue^*,1, John Perea², Steven Theg², ¹ Department of Plant Sciences, University of California, Davis, CA, USA² Section of Plant Biology, University of California, Davis, CA, USA

ABSTRACT- Toc75 is the protein translocation channel in the outer envelope of the chloroplast, and is essential for viability of plants from the stage of embryogenesis. It is encoded by the nuclear genome, thus has to be targeted to the organelle posttranslationally. Toc75 is the only known protein in the outer membrane of the mitochondrion or the chloroplastic envelope that is synthesized with an N-terminal extension, or a transit peptide, which is removed during or after the protein is targeted to and assembled in the membrane. Interestingly, two alanine residues are found at -3 and -1 to the cleavage site, which is similar to the sequence recognized and cleaved by the type I signal peptidase (SPase I). In order to understand the mechanism for maturation of Toc75, we sought to identify a peptidase that can catalyze the removal of its transit peptide. First, we incubated the radio-labeled precursor form of Toc75 (pr75) with Lep, a SPase I from Escherichia coli, and found that pr75 was efficiently converted to the mature form of Toc75 (m75). This result led us to the next step, which was to examine whether or not any SPase I-like protein in plants is responsible for maturation of Toc75. To this end, we identified a cDNA from Arabidopsis encoding the plastidic signal peptidase (Plsp), a SPase I homolog with a putative chloroplast targeting signal. The presence of the Plsp protein in chloroplasts was confirmed by a recent proteomic study. Knockout of PLSP by a T-DNA insertion resulted in lack of accumulation of m75 and in a seedling lethal phenotype. These results suggest that 1) Plsp is responsible for maturation of Toc75; 2) maturation of Toc75 is not essential for embryogenesis, but may be required for biogenesis of chloroplasts. Part of the work is supported by USDA-CSREES 2003-02860 to KI.

KEY WORDS: Toc75, Signal peptidase, Chloroplast biogenesis, Protein translocation