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Published 10 May 2004. doi:10.1083/jcb.200311074
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 165, Number 3, 323-334
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Article

atToc159 is a selective transit peptide receptor for the import of nucleus-encoded chloroplast proteins



Matthew D. Smith1, Caleb M. Rounds1, Fei Wang1, Kunhua Chen2, Meshack Afitlhile1, and Danny J. Schnell1

1 Department of Biochemistry and Molecular Biology and Program in Plant Biology, University of Massachusetts, Amherst, MA 01003
2 Plant Molecular and Cellular Biology Laboratory, The Salk Institute, La Jolla, CA 92037

Address correspondence to Danny J. Schnell, Dept. of Biochemistry and Molecular Biology, 820 LGRC, University of Massachusetts, Amherst, MA 01003. Tel.: (413) 545-4024. Fax: (413) 545-3291. email: dschnell{at}biochem.umass.edu

The members of the Toc159 family of GTPases act as the primary receptors for the import of nucleus-encoded preproteins into plastids. Toc159, the most abundant member of this family in chloroplasts, is required for chloroplast biogenesis (Bauer, J., K. Chen, A. Hiltbunner, E. Wehrli, M. Eugster, D. Schnell, and F. Kessler. 2000. Nature. 403:203–207) and has been shown to covalently cross-link to bound preproteins at the chloroplast surface (Ma, Y., A. Kouranov, S. LaSala, and D.J. Schnell. 1996. J. Cell Biol. 134:1–13; Perry, S.E., and K. Keegstra. 1994. Plant Cell. 6:93–105). These reports led to the hypothesis that Toc159 functions as a selective import receptor for preproteins that are required for chloroplast development. In this report, we provide evidence that Toc159 is required for the import of several highly expressed photosynthetic preproteins in vivo. Furthermore, we demonstrate that the cytoplasmic and recombinant forms of soluble Toc159 bind directly and selectively to the transit peptides of these representative photosynthetic preproteins, but not representative constitutively expressed plastid preproteins. These data support the function of Toc159 as a selective import receptor for the targeting of a set of preproteins required for chloroplast biogenesis.

Key Words: chloroplasts; protein transport; protein binding; cross-linking; GTPase

M. Afitlhile's present address is Kentucky State University, 400 East Main St., Frankfort, KY 40601.

Abbreviations used in this paper: DHFR, dihydrofolate reductase; GMP-PNP, guanyl-5'-yl imidodiphosphate; Ni-NTA, nickel-nitrilotriacetic acid; pE1{alpha}, precursor to the E1{alpha} subunit of plastid pyruvate dehydrogenase; PEAS, N-((2-pyridyldithio)ethyl)-4-azidosalicylamide; pL11, precursor to the plastid ribosomal L11 subunit; pPORA, precursor to protochlorophyllide oxidoreductase A; protA, protein A; pSSU, precursor to the small subunit of Rubisco; Toc, translocon at the outer envelope membrane of chloroplasts.


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