Ion homeostasis in the Chloroplast

peer reviewedThe chloroplast is an organelle of high demand for macro- and micro-nutrient ions, which are required for the maintenance of the photosynthetic process. To avoid deficiency while preventing excess, homeostasis mechanisms must be tightly regulated. Here, we describe the needs for nutrient ions in the chloroplast and briefly highlight their functions in the chloroplastidial metabolism. We further discuss the impact of nutrient deficiency on chloroplasts and the acclimation mechanisms that evolved to preserve the photosynthetic apparatus. We finally present what is known about import and export mechanisms for these ions. Whenever possible, a comparison between cyanobacteria, algae and plants is provided to add an evolutionary perspective to the description of ion homeostasis mechanisms in photosynthesis

The sulfur acclimation SAC3 kinase is required for chloroplast transcriptional repression under sulfur limitation in Chlamydomonas reinhardtii

Sulfur (S) deprivation responses have been studied extensively in algae and land plants; however, little is known of the signals that link perception of S status to chloroplast gene expression. Here, we have compared the chloroplast S limitation response in WT vs. sac1 and sac3 sulfur acclimation mutants of the green alga Chlamydomonas reinhardtii. We provide evidence that in the WT, chloroplast transcriptional activity rapidly decreases after removal of S from the medium, leading to reduced transcript accumulation. This decrease correlates with reduced abundance of a σ(70)-like factor, Sig1, which is most likely the unique chloroplast transcription specificity factor. We further show that reduced transcription activity and diminished Sig1 accumulation are mediated by the SAC3 gene product, a putative Snf1-type Ser/Thr kinase previously shown to have both positive and negative effects on nuclear gene expression. Inclusion of the protein kinase inhibitor 6-dimethylaminopurine during S limitation yielded a pattern of expression that was largely similar to that seen in the sac3 mutant, lending support to the hypothesis that Sac3 kinase activation leads to transcriptional repression and Sig1 proteolysis. The finding that Sac3 regulates chloroplast gene expression suggests that it has a previously unknown role in integrating the S limitation response in multiple subcellular compartments

X-ray structure of a protein-conducting channel

A conserved heterotrimeric membrane protein complex, the Sec61 or SecY complex, forms a protein-conducting channel, allowing polypeptides to be transferred across or integrated into membranes. We report the crystal structure of the complex from Methanococcus jannaschii at a resolution of 3.2 Å. The structure suggests that one copy of the heterotrimer serves as a functional translocation channel. The α-subunit has two linked halves, transmembrane segments 1–5 and 6–10, clamped together by the γ-subunit. A cytoplasmic funnel leading into the channel is plugged by a short helix. Plug displacement can open the channel into an ‘hourglass’ with a ring of hydrophobic residues at its constriction. This ring may form a seal around the translocating polypeptide, hindering the permeation of other molecules. The structure also suggests mechanisms for signal-sequence recognition and for the lateral exit of transmembrane segments of nascent membrane proteins into lipid, and indicates binding sites for partners that provide the driving force for translocation