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© The Rockefeller University Press, 0021-9525/1997//1279 $5.00
The Journal of Cell Biology, Volume 137, Number 6, , 1997 1279-1286

Article

A Nuclear-coded Chloroplastic Inner Envelope Membrane Protein Uses a Soluble Sorting Intermediate upon Import into the Organelle



Jens Lübeck, Lisa Heins, and Jürgen Soll

Botanisches Institut, Universität Kiel, 24118 Kiel, Germany

The chloroplastic inner envelope protein of 110 kD (IEP110) is part of the protein import machinery in the pea. Different hybrid proteins were constructed to assess the import and sorting pathway of IEP110. The IEP110 precursor (pIEP110) uses the general import pathway into chloroplasts, as shown by the mutual exchange of presequences with the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSSU). Sorting information to the chloroplastic inner envelope is contained in an NH2-proximal part of mature IEP110 (110N). The NH2-terminus serves to anchor the protein into the membrane. Large COOH-terminal portions of this protein (80–90 kD) are exposed to the intermembrane space in situ. Successful sorting and integration of IEP110 and the derived constructs into the inner envelope are demonstrated by the inaccessability of processed mature protein to the protease thermolysin but accessibility to trypsin, i.e., the imported protein is exposed to the intermembrane space. A hybrid protein consisting of the transit sequence of SSU, the NH2-proximal part of mature IEP110, and mature SSU (tpSSU-110N-mSSU) is completely imported into the chloroplast stroma, from which it can be recovered as soluble, terminally processed 110NmSSU. The soluble 110N-mSSU then enters a reexport pathway, which results not only in the insertion of 110N-mSSU into the inner envelope membrane, but also in the extrusion of large portions of the protein into the intermembrane space. We conclude that chloroplasts possess a protein reexport machinery for IEPs in which soluble stromal components interact with a membrane-localized translocation machinery.

Abbreviations used in this paper: chl, chlorophyll; IEP, inner envelope protein; IEP110, chloroplastic IEP of 110 kD; m, mature form of; p, precursor of; SSU, small subunit of ribulose-1,5-bisphosphate carboxylase; tp, transit peptide of; 110N, NH2-proximal part of mature IEP110.

Address all correspondence to Professor Jürgen Soll, Botanisches Institut, Christian-Albrechts-Universität, D-24118 Kiel, Germany. Tel.: (49) 431880-4210. Fax: (49) 431-880-1527. E-mail: jsoll{at}bot.uni-kiel.de


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