TAAC is readily expressed in dark-grown Arabidopsis seedlings, and its level remains stable throughout the greening process. Its expression is highest in developing green tissues and in leaves undergoing senescence or abiotic stress. We propose that the TAAC protein supplies ATP for energy-dependent reactions during thylakoid biogenesis and turnover in plants.Chloroplasts perform oxygenic photosynthesis in algae and plants and have evolved by endosymbiosis from cyanobacteria. Chloroplasts have two distinct membrane systems, the double envelope surrounding the organelle and an internal membrane system named thylakoids. The envelope membrane represents the interface between the cytoplasm and chloroplast stroma, whereas the thylakoid membrane separates the stroma and the lumenal space. Altogether ϳ800 membrane proteins have been identified by proteomics in the envelope and thylakoid membranes of Arabidopsis thaliana (for reviews, see Refs. 1 and 2). As expected, the main function for the identified envelope proteins was transport of ions and metabolites, whereas photosynthesis was attributed to most of the identified thylakoid proteins. The major protein complexes in thylakoids are photosystems (PS) 4 I and II, the cytochrome b 6 f complex, and the proton-translocating ATP synthase. These photosynthetic complexes contain not only proteins but also pigments and other cofactors. Their assembly, activity, and removal require a large number of auxiliary, regulatory, and transport proteins (3, 4). Many biochemical reports pointed to the existence of transport activities in the thylakoid membrane, such as calcium transport (5), copper transport (6), anion channels (7), cation channels (8, 9), and nucleotide transport (10). Only the thylakoid copper transporter was identified at the genetic level in Arabidopsis (11). No hydrophobic proteins related to the above-mentioned transport activities were identified in the previous proteomic works on Arabidopsis thylakoid membranes (for a review, see Ref.2). Therefore, genetic strategies are required for identification and elucidation of their role in optimal function of the thylakoid.ATP is produced during the light-dependent photosynthetic reactions on the stromal side of the thylakoid membrane. Besides its utilization during CO 2 fixation in the stroma, ATP drives many energy-dependent processes in thylakoids, including protein phosphorylation, folding, import, and degradation.

The Arabidopsis APETALA3 (AP3) and PISTILLATA (PI) proteins are thought to act as transcription Jactors and are required for specifying floral organ identities. To define the nuclear localization signals within these proteins, we generated translational fusions of the coding regions of AP3 and PI to the bacterial uidA gene that encodes p-glucuronidase (GUS). Transient transformation assays of either the AP3-GUS or PI-GUS fusion protein alone resulted in cytoplasmic localization of GUS activity. However, coexpression of AP3-GUS with PI, or PI-GUS with APS, resulted in nuclear localization of GUS activity. Stable transformation with these fusion proteins in Arabidopsis showed similar results. The nuclear colocalization signals in AP3 and PI were mapped to the amino-terminal regions of each protein. These observations suggest that the interaction of the AP3 and PI gene products results in the formation of a protein complex that generates or exposes a colocalization signal required to translocate the resulting complex into the nucleus. The colocalization phenomenon that we have described represents a novel mechanism to coordinate the functions of transcription factors within the nucleus.

Reticulons are proteins that have been found predominantly associated with the endoplasmic reticulum in yeast and mammalian cells. While their functions are still poorly understood, recent findings suggest that they participate in the shaping of the tubular endoplamic reticulum (ER). Although reticulonlike proteins have been identified in plants, very little is known about their cellular localization and functions. Here, we characterized the reticulon-like protein family of Arabidopsis thaliana. Three subfamilies can be distinguished on the basis of structural organization and sequence homology. We investigated the subcellular localization of two members of the largest subfamily, i.e. AtRTNLB2 and AtRTNLB4, using fluorescent protein tags. The results demonstrate for the first time that plant reticulonlike proteins are associated with the ER. Both AtRTNLB proteins are located in the tubular ER but AtRTNLB4 is also found in the lamellar ER cisternae, and in ER tubules in close association with the chloroplasts. Similarity in protein structure and subcellular localization between AtRTNLB2 and mammalian reticulons suggests that they could assume similar basic functions inside the cell.