SORTING OF NUCLEAR-ENCODED CHLOROPLAST MEMBRANE-PROTEINS TO THE ENVELOPE AND THE THYLAKOID MEMBRANE

The spinach triose phosphate/phosphate translocator and the 37-kDa pro tein are both integral components of the chloroplast inner envelope me mbrane. They are synthesized in the cytosol with N-terminal extensions , the transit peptides, that are different in structural terms from th ose of imported stromal or thylakoid proteins. In order to determine i f these N-terminal extensions are essential for the correct localizati on to the envelope membrane, they were linked to the mature parts of t hylakoid membrane proteins, the light-harvesting chlorophyll a/b bindi ng protein and the CF0II-subunit of the thylakoid ATP synthase, respec tively. In addition, the transit peptide of the CF0II-subunit that con tains signals for the transport across both the envelope and the thyla koid membrane was fused to the mature parts of both envelope membrane proteins. The chimeric proteins were imported into isolated spinach ch loroplasts, and the intraorganellar routing of the proteins was analyz ed. The results obtained show that the N-terminal extensions of both e nvelope membrane proteins possess a stroma-targeting function only and that the information for the integration into the envelope membrane i s contained in the mature parts of the proteins. At least part of the integration signal is provided by hydrophobic domains in the mature se quences since the removal of such a hydrophobic segment from the 37-kD a protein leads to missorting of the protein to the stroma and the thy lakoid membrane.