On the mode of integration of plastid-encoded components of the cytochrome bf complex into thylakoid membranes

• Published in: Planta 1997-03, Vol.201 (3), p.334-341
• Main Authors: Zak, E, Sokolenko, A, Unterholzner, G, Altschmied, L, Herrmann, R.G. (Muenchen Univ. (Germany). Botanisches Inst.)
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Cell biochemistry; CELL MEMBRANES; Cell physiology; Cell structures and functions; Chloroplast, photosynthetic membrane and photosynthesis; CHLOROPLASTE; CHLOROPLASTS; CITOCROMOS; CLOROPLASTO; CYTOCHROME; CYTOCHROMES; Epitopes; EVOLUCION; EVOLUTION; FOTOSINTESIS; Fundamental and applied biological sciences. Psychology; GENE; GENES; Hydrophobicity; Imports; Integration; MEMBRANAS CELULARES; MEMBRANE CELLULAIRE; Membranes; Mitochondrial DNA; Molecular and cellular biology; NOYAU CELLULAIRE; NUCLEO; NUCLEUS; Organelles; PEPTIDE; PEPTIDES; PEPTIDOS; PHOTOSYNTHESE; PHOTOSYNTHESIS; PISUM SATIVUM; Plant physiology and development; PLANTULAS; PLANTULE; Plasmamembran; PLASTE; PLASTIDIOS; PLASTIDS; Polypeptid; Polypeptides; Post-translation; Protein precursors; PROTEINAS; PROTEINE; PROTEINS; Public transportation; SEEDLINGS; Segments; Spinach; SPINACIA OLERACEA; Thylakoid; Thylakoid membranes; Thylakoids; Translocation
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s004250050075

Four distinct integration/translocation routes into/across thylakoid membranes have recently been deduced for nuclear-encoded polypeptides of the photosynthetic membrane. Corresponding information for the plastid-encoded protein complement is lacking. We have investigated this aspect with in-organello assays employing chimeric constructs generated with codon-correct cassettes for genes of plastid-encoded thylakoid proteins, and appropriate transit peptides from six nuclear genes, representing three targeting classes, as a strategy. The three major plastid-encoded components of the cytochrome b6f complex, namely pre-apocytochrome f, (including apocytochrome f, and pre-apocytochrome f, lacking the C-terminal transmembrane segment), cytochrome b6, and subunit IV, which differ in the number of their transmembrane segments, were studied. Import into chloroplasts could be observed in all instances but with relatively low efficiency. Thylakoid integration can occur post-translationally, but only components with secretory/secretory pathway (SEC)-route-specific epitopes were correctly assembled with the cytochrome complex, or competed with this process. Inhibitor studies were consistent with these findings. Imported cytochrome b6 and subunit IV operated with uncleaved targeting signals for thylakoid integration. The corresponding determinant for cytochrome f is its signal peptide; its C-terminal hydrophobic segment did not, or did not appreciably, contribute to this process. The N-termini of cytochrome b6 and subunit IV appear to reside on the same (lumenal) side of the membrane, consistent with the currently favored four-helix model for the cytochrome, but in disagreement with the topography proposed for both components. The impact of the findings for protein routing, including for applied approaches such as compartment-alien transformation, is discussed.