Without a little help from 'my' friends: direct insertion of proteins into chloroplast membranes?

A GTP-driven motor moves proteins across the outer envelope of chloroplasts

The translocation of proteins across cellular membranes is a key mechanistic problem for every cell. The preprotein translocon at the chloroplast outer envelope is responsible for precursor protein recognition and translocation across the outer envelope. We have reconstituted the translocation process into proteoliposomes from single subunits or by using the purified translocon. Precursor proteins are recognized by the Toc34 receptor in an initial GTP-dependent process. Translocation across the plane of the membrane then occurs through the Toc75 channel in a GTP-dependent process. Correspondingly, GTP hydrolysis of Toc proteoliposomes is 100-fold enhanced in the presence of preprotein. Complete translocation is demonstrated by processing of the precursor form to the mature form by the stromal processing peptidase and by protease resistance of the imported protein. Molecular chaperones are not involved in this translocation event. We show that Toc159 acts as a GTP-driven motor in a sewing-machine-like mechanism.

Prediction of the plant beta-barrel proteome: a case study of the chloroplast outer envelope

In the postgenomic era, the transformation of genetic information into biochemical meaning is required. We have analyzed the proteome of the chloroplast outer envelope membrane by an in silico and a proteomic approach. Based on its evolutionary relation to the outer membrane of Gram-negative bacteria, the outer envelope membrane should contain a large number of beta-barrel proteins. We therefore calculated the probability for the existence of beta-sheet, beta-barrel, and hairpin structures among all proteins of the Arabidopsis thaliana genome. According to the existence of these structures, a number of candidates were selected. This protein pool was analyzed by TargetP to discard sequences with signals that would direct the protein to other organelles different from chloroplasts. In addition, the pool was manually controlled for the presence of proteins known to function outside of the chloroplast envelope. The approach developed here can be used to predict the topology of beta-barrel proteins. For the proteomic approach, proteins of highly purified outer envelope membranes of chloroplasts from Pisum sativum were analyzed by ESI-MS/MS mass spectrometry. In addition to the known components, four new proteins of the outer envelope membranes were identified in this study.

Characterization of the translocon of the outer envelope of chloroplasts

The protein translocon of the outer envelope of chloroplasts (Toc) consists of the core subunits Toc159, Toc75, and Toc34. To investigate the molecular structure, the core complex was purified. This core complex has an apparent molecular mass of approximately 500 kD and a molecular stoichiometry of 1:4:4-5 between Toc159, Toc75, and Toc34. The isolated translocon recognizes both transit sequences and precursor proteins in a GTP-dependent manner, suggesting its functional integrity. The complex is embedded by the lipids phosphatidylcholine and digalactosyldiacylglyceride. Two-dimensional structural analysis by EM revealed roughly circular particles consistent with the formation of a stable core complex. The particles show a diameter of approximately 130 A with a solid ring and a less dense interior structure. A three-dimensional map obtained by random conical tilt reconstruction of electron micrographs suggests that a "finger"-like central region separates four curved translocation channels within one complex.

Genomic reduction and evolution of novel genetic membranes and protein-targeting machinery in eukaryote-eukaryote chimaeras (meta-algae)

Chloroplasts originated just once, from cyanobacteria enslaved by a biciliate protozoan to form the plant kingdom (green plants, red and glaucophyte algae), but subsequently, were laterally transferred to other lineages to form eukaryote-eukaryote chimaeras or meta-algae. This process of secondary symbiogenesis (permanent merger of two phylogenetically distinct eukaryote cells) has left remarkable traces of its evolutionary role in the more complex topology of the membranes surrounding all non-plant (meta-algal) chloroplasts. It took place twice, soon after green and red algae diverged over 550 Myr ago to form two independent major branches of the eukaryotic tree (chromalveolates and cabozoa), comprising both meta-algae and numerous secondarily non-photosynthetic lineages. In both cases, enslavement probably began by evolving a novel targeting of endomembrane vesicles to the perialgal vacuole to implant host porter proteins for extracting photosynthate. Chromalveolates arose by such enslavement of a unicellular red alga and evolution of chlorophyll c to form the kingdom Chromista and protozoan infrakingdom Alveolata, which diverged from the ancestral chromalveolate chimaera. Cabozoa arose when the common ancestor of euglenoids and cercozoan chlorarachnean algae enslaved a tetraphyte green alga with chlorophyll a and b. I suggest that in cabozoa the endomembrane vesicles originally budded from the Golgi, whereas in chromalveolates they budded from the endoplasmic reticulum (ER) independently of Golgi-targeted vesicles, presenting a potentially novel target for drugs against alveolate Sporozoa such as malaria parasites and Toxoplasma. These hypothetical ER-derived vesicles mediated fusion of the perialgal vacuole and rough ER (RER) in the ancestral chromist, placing the former red alga within the RER lumen. Subsequently, this chimaera diverged to form cryptomonads, which retained the red algal nucleus as a nucleomorph (NM) with approximately 464 protein-coding genes (30 encoding plastid proteins) and a red or blue phycobiliprotein antenna pigment, and the chromobiotes (heterokonts and haptophytes), which lost phycobilins and evolved the brown carotenoid fucoxanthin that colours brown seaweeds, diatoms and haptophytes. Chromobiotes transferred the 30 genes to the nucleus and lost the NM genome and nuclear-pore complexes, but retained its membrane as the periplastid reticulum (PPR), putatively the phospholipid factory of the periplastid space (former algal cytoplasm), as did the ancestral alveolate independently. The chlorarachnean NM has three minute chromosomes bearing approximately 300 genes riddled with pygmy introns. I propose that the periplastid membrane (PPM, the former algal plasma membrane) of chromalveolates, and possibly chlorarachneans, grows by fusion of vesicles emanating from the NM envelope or PPR. Dinoflagellates and euglenoids independently lost the PPM and PPR (after diverging from Sporozoa and chlorarachneans, respectively) and evolved triple chloroplast envelopes comprising the original plant double envelope and an extra outermost membrane, the EM, derived from the perialgal vacuole. In all metaalgae most chloroplast proteins are coded by nuclear genes and enter the chloroplast by using bipartite targeting sequences--an upstream signal sequence for entering the ER and a downstream chloroplast transit sequence. I present a new theory for the four-fold diversification of the chloroplast OM protein translocon following its insertion into the PPM to facilitate protein translocation across it (of both periplastid and plastid proteins). I discuss evidence from genome sequencing and other sources on the contrasting modes of protein targeting, cellular integration, and evolution of these two major lineages of eukaryote "cells within cells". They also provide powerful evidence for natural selection's effectiveness in eliminating most functionless DNA and therefore of a universally useful non-genic function for nuclear non-coding DNA, i.e. most DNA in the biosphere, and dramatic examples of genomic reduction. I briefly argue that chloroplast replacement in dinoflagellates, which happened at least twice, may have been evolutionarily easier than secondary symbiogenesis because parts of the chromalveolate protein-targeting machinery could have helped enslave the foreign plastids.

Essential role of the G-domain in targeting of the protein import receptor atToc159 to the chloroplast outer membrane

Two homologous GTP-binding proteins, atToc33 and atToc159, control access of cytosolic precursor proteins to the chloroplast. atToc33 is a constitutive outer chloroplast membrane protein, whereas the precursor receptor atToc159 also exists in a soluble, cytosolic form. This suggests that atToc159 may be able to switch between a soluble and an integral membrane form. By transient expression of GFP fusion proteins, mutant analysis, and biochemical experimentation, we demonstrate that the GTP-binding domain regulates the targeting of cytosolic atToc159 to the chloroplast and mediates the switch between cytosolic and integral membrane forms. Mutant atToc159, unable to bind GTP, does not reinstate a green phenotype in an albino mutant (ppi2) lacking endogenous atToc159, remaining trapped in the cytosol. Thus, the function of atToc159 in chloroplast biogenesis is dependent on an intrinsic GTP-regulated switch that controls localization of the receptor to the chloroplast envelope.

Topology of polytopic membrane protein subdomains is dictated by membrane phospholipid composition

In Escherichia coli, the major cytoplasmic domain (C6) of the polytopic membrane protein lactose permease (LacY) is exposed to the opposite side of the membrane from a neighboring periplasmic domain (P7). However, these domains are both exposed on the periplasmic side of the membrane in a mutant of E.coli lacking phosphatidylethanolamine (PE) wherein LacY only mediates facilitated transport. When purified LacY was reconstituted into liposomes lacking PE or phosphatidylcholine (PC), C6 and P7 were on the same side of the bilayer. In liposomes containing PE or PC, C6 and P7 were on opposite sides of the bilayer. Only the presence of PE in the liposomes restored active transport function of LacY as opposed to restoration of only facilitated transport function in the absence of PE. These results were the same for LacY purified from PE-containing or PE-lacking cells, and are consistent with the topology and function of LacY assembled in vivo. Therefore, irrespective of the mechanism of membrane insertion, the subdomain topological orientation and function of LacY are determined primarily by membrane phospholipid composition.

The predicted candidates of Arabidopsis plastid inner envelope membrane proteins and their expression profiles

Plastid envelope proteins from the Arabidopsis nuclear genome were predicted using computational methods. Selection criteria were: first, to find proteins with NH(2)-terminal plastid-targeting peptides from all annotated open reading frames from Arabidopsis; second, to search for proteins with membrane-spanning domains among the predicted plastidial-targeted proteins; and third, to subtract known thylakoid membrane proteins. Five hundred forty-one proteins were selected as potential candidates of the Arabidopsis plastid inner envelope membrane proteins (AtPEM candidates). Only 34% (183) of the AtPEM candidates could be assigned to putative functions based on sequence similarity to proteins of known function (compared with the 69% function assignment of the total predicted proteins in the genome). Of the 183 candidates with assigned functions, 40% were classified in the category of "transport facilitation," indicating that this collection is highly enriched in membrane transporters. Information on the predicted proteins, tissue expression data from expressed sequence tags and microarrays, and publicly available T-DNA insertion lines were collected. The data set complements proteomic-based efforts in the increased detection of integral membrane proteins, low-abundance proteins, or those not expressed in tissues selected for proteomic analysis. Digital northern analysis of expressed sequence tags suggested that the transcript levels of most AtPEM candidates were relatively constant among different tissues in contrast to stroma and the thylakoid proteins. However, both digital northern and microarray analyses identified a number of AtPEM candidates with tissue-specific expression patterns.

Chloroplast quest: a journey from the cytosol into the chloroplast and beyond

Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.

Chloroplast SecE: evidence for spontaneous insertion into the thylakoid membrane

SecE, an essential component of the bacterial SecAYEG translocase, mediates protein translocation across the cytoplasmic membrane. In the thylakoid membranes of chloroplasts an SecE homologue, cpSecE, has recently been identified. In this report we show that insertion of cpSecE does not require stromal extract, indicating that signal recognition particle is not involved. Removal of nucleoside triphosphates has apparently no effect on the integration, again ruling out an involvement of SRP or its partner protein, FtsY. The use of well-known inhibitors of the Sec- and Tat pathways, sodium azide and nigericin, respectively, also had no influence on membrane insertion. The data presented here point towards cpSecE as another passenger of a wholly spontaneous import/insertion pathway in the thylakoids of chloroplasts.