Discovery of a protein required for photosynthetic membrane assembly.

A one-half of the '57 proteins of the photosynthetic membranes in plant chloroplasts is encoded in the chloroplast genome, translated on chloroplast ribosomes and cotranslationally inserted into the lipid bilayer of the thylakoids (flattened sacs; ref. 1). The other half is translated on cytosolic ribosomes from messenger RNA of nuclear genes and posttranslationally imported across the outer and inner chloroplast envelope membranes. The import machinery consists of smart receptor proteins, translocation channels, and chaperon proteins (2, 3). The photosynthetic proteins are assembled into five multisubunit complexes. There, the pigments for light harvesting and the electron transport molecules for the primary light-dependent charge separation and for the generation of protons and oxygen in the water-splitting reaction are positioned at the optimal distances. The two photosystems cooperate via the cytochrome b6f complex in the transfer of electrons to provide reducing power in the form of NADPH and establishment of proton gradients from the stroma side to the lumen of the thylakoid. The electrochemical gradient formed by the accumulation of protons in the thylakoid lumen provides the driving force for the phosphorylation of ADP by the ATP synthase complex. In contrast to the profound knowledge of the organization of the photosynthetic membrane and the import of the protein components into the chloroplast and their targeting to the thylakoids, progress in learning how the lipid bilayer membranes are formed is less obvious. This situation may change with the discovery of a function of the vesicleinducing protein in plastids (VIPP) by Kroll et al. and Westphal et al. as reported in this issue of PNAS (4, 5). In pea chloroplasts, the 37-kDa VIPP protein is located both in the vicinity of the chloroplast envelope and in the thylakoid membranes (6) and was considered by Li, Kaneko, and Keegstra (6) as a candidate for the transfer of galactolipids from their site of synthesis at the chloroplast envelope to the thylakoids. Daniela Kroll, Karin Meierhoff, Nicole Bechtold, Mikio Kinoshita, Sabine Westphal, Ute Vothknecht, Jürgen Soll, and Peter Westhoff studied a recessive Arabidosis T-DNA [portion of the Ti (tumor-inducing) plasmid that is transferred to plant cells] insertion mutant with severe disturbances in the photosynthetic electron transport chain and the formation of the thylakoids. The insertion was identified in the gene encoding VIPP, and the mutant could be rescued by transformation with the VIPP cDNA. The cause for the disturbed development or maintenance of the thylakoids was the failure of the mutant to bud from the inner chloroplast envelope membrane vesicles, which transfer lipids from the inner envelope to the thylakoid membranes (7–10). In the transformants, the process of vesicle budding was reestablished, and the thylakoid organization normalized. The companion paper by Sabine Westphal, Lisa Heins, Jürgen Soll, and Ute Vothknecht identifies VIPP 1 genes in the genomes of Synechocystis, Anabaena, Synechococcus, and Nostoc. In these cyanobacteria, the protein is located in the plasma membrane, but its disruption in Synechocystis by insertion mutagenesis with a kanamycin cassette prevents ordered thylakoid formation and light-dependent oxygen evolution. The protein has high amino acid sequence identity with the PspA protein of Escherichia coli, but it has evolved by addition of a novel C-terminal domain of some 50 aa. In E. coli, this protein is part of an operon induced by infection with the filamentous phage f1 and by treatments that inhibit the protein translocation pathway via the Sec pathway, which is also used in chloroplasts and cyanobacteria for translocation of proteins across the thylakoid membranes. This partial identity indicates a gene duplication and recruitment for the novel function during evolution. Is the vesicle transport responsible for the establishment of the lipid bilayers? In higher plants, chloroplasts develop from proplastids in the light or via the etioplast pathway after an initial dark period (Fig. 1). The