Chloroplast Protein Import: Reverse Genetic Approaches

The outer membrane Omp85-like protein P39 influences metabolic homeostasis in mature Arabidopsis thaliana

The Omp85 proteins form a large membrane protein family in bacteria and eukaryotes. Omp85 proteins are composed of a C-terminal β-barrel-shaped membrane domain and one or more N-terminal polypeptide transport-associated (POTRA) domains. However, Arabidopsis thaliana contains two genes coding for Omp85 proteins without a POTRA domain. One gene is designated P39, according to the molecular weight of the encoded protein. The protein is targeted to plastids and it was established that p39 has electrophysiological properties similar to other Omp85 family members, particularly to that designated as Toc75V/Oep80. We analysed expression of the gene and characterised two T-DNA insertion mutants, focusing on alterations in photosynthetic activity, plastid ultrastructure, global expression profile and metabolome. We observed pronounced expression of P39, especially in veins. Mutants of P39 show growth aberrations, reduced photosynthetic activity and changes in plastid ultrastructure, particularly in the leaf tip. Further, they display global alteration of gene expression and metabolite content in leaves of mature plants. We conclude that the function of the plastid-localised and vein-specific Omp85 family protein p39 is important, but not essential, for maintenance of metabolic homeostasis of full-grown A. thaliana plants. Further, the function of p39 in veins influences the functionality of other plant tissues. The link connecting p39 function with metabolic regulation in mature A. thaliana is discussed.

The Omp85-type outer membrane protein p36 of Arabidopsis thaliana evolved by recent gene duplication

Proteins of the Omp85 family are involved in the insertion of β-barrel shaped outer membrane proteins in bacteria and mitochondria, and-at least-in the transfer of preproteins across the chloroplast outer envelope. In general these proteins consist of up to five N-terminal "polypeptide transport associated" (POTRA) domains and a C-terminal, membrane embedded β-barrel domain. In Arabidopsis thaliana two plastidic gene families coding for Omp85-like proteins exist, namely the Toc75-III and the Toc75-V/Oep80 sub-family. The latter is composed of three genes, of which two do not contain POTRA domains. These are annotated as P39 and P36. However, P36 resulted from a very recent gene duplication of P39 and appears to be specific to Arabidopsis thaliana. Furthermore, we show that P39 is specifically expressed in vein tissues, while P36 is expressed at early and late developmental stages. T-DNA insertion in P36 causes a mild phenotype with reduced starch accumulation in chloroplasts of sepals pointing towards a yet to be described plastid function.

Toc33 and Toc64-III cooperate in precursor protein import into the chloroplasts of Arabidopsis thaliana

The import of cytosolically synthesized precursor proteins into chloroplasts by the translocon at the outer envelope membrane of chloroplasts (TOC) is crucial for organelle function. The recognition of precursor proteins at the chloroplast surface precedes translocation and involves the membrane-inserted receptor subunits Toc34 and Toc159. A third receptor, Toc64, was discussed to recognize cytosolic complexes guiding precursor proteins to the membrane surface, but this function remains debated. We analysed Arabidopsis thaliana plants carrying a T-DNA insertion in the gene encoding the Toc64 homolog Toc64-III. We observed a light intensity-dependent growth phenotype, which is distinct from the phenotype of ppi1, the previously described mutant of the TOC34 homolog TOC33. Furthermore, chloroplast import of the model precursor proteins pOE33 and pSSU into chloroplasts is reduced in protoplasts isolated from plants with impaired Toc64-III function. This suggests that Toc64-III modulates the translocation efficiency in vivo. A ppi1 and toc64-III double mutant shows a significant increase in the transcript levels of HSP90 and TOC75-III, the latter coding for the pore-forming TOC component. Remarkably, the protein level of Toc75-III is significantly reduced, suggesting that Toc64-III and Toc33 cooperate in the insertion or stabilization of Toc75-III. Accordingly, the results presented support Toc64 as an import-relevant component of the TOC complex.

In vivo studies on the roles of Tic55-related proteins in chloroplast protein import in Arabidopsis thaliana

The Tic55 (Translocon at the inner envelope membrane of chloroplasts, 55 kDa) protein was identified in pea as a putative regulator, possibly linking chloroplast protein import to the redox state of the photosynthetic machinery. Two Tic55 homologs have been proposed to exist in Arabidopsis: atTic55-II and AtPTC52 (Protochlorophyllide-dependent Translocon Component, 52 kDa; has also been called atTic55-IV). Our phylogenetic analysis shows that atTic55-II is an ortholog of psTic55 from pea (Pisum sativum), and that AtPTC52 is a more distant homolog of the two. AtPTC52 was included in this study to rule out possible functional links between the proteins in Arabidopsis. No detectable mutant phenotypes were found in two independent T-DNA knockout mutant plant lines for each Arabidopsis protein, when compared with wild-type: visible appearance, chlorophyll content, photosynthetic performance, and chloroplast protein import, for example, were all normal. Both wild-type and tic55-II mutant chloroplasts exhibited deficient protein import when treated with diethylpyrocarbonate, indicating that Tic55 is not the sole target of this reagent in relation to protein import. Furthermore, ptc52 mutant chloroplasts were not defective with respect to pPORA import, which was previously reported to involve PTC52 in barley. Thus, we conclude that atTic55-II and AtPTC52 are not strictly required for functional protein import in Arabidopsis.

Phosphorylation regulates the assembly of chloroplast import machinery

Chloroplast function depends on the translocation of cytosolically synthesized precursor proteins into the organelle. The recognition and transfer of most precursor proteins across the outer membrane depend on a membrane inserted complex. Two receptor components of this complex, Toc34 and Toc159, are GTPases, which can be phosphorylated by kinases present in the hosting membrane. However, the physiological function of phosphorylation is not yet understood in detail. It is demonstrated that both receptors are phosphorylated within their G-domains. In vitro, the phosphorylation of Toc34 disrupts both homo- and heterodimerization of the G-domains as determined using a phospho-mimicking mutant. In endogenous membranes this mutation or phosphorylation of the wild-type receptor disturbs the association of Toc34, but not of Toc159 with the translocation pore. Therefore, phosphorylation serves as an inhibitor for the association of Toc34 with other components of the complex and phosphorylation can now be discussed as a mechanism to exchange different isoforms of Toc34 within this ensemble.

The GTPase cycle of the chloroplast import receptors Toc33/Toc34: implications from monomeric and dimeric structures

Transport of precursor proteins across chloroplast membranes involves the GTPases Toc33/34 and Toc159 at the outer chloroplast envelope. The small GTPase Toc33/34 can homodimerize, but the regulation of this interaction has remained elusive. We show that dimerization is independent of nucleotide loading state, based on crystal structures of dimeric Pisum sativum Toc34 and monomeric Arabidopsis thaliana Toc33. An arginine residue is--in the dimer--positioned to resemble a GAP arginine finger. However, GTPase activation by dimerization is sparse and active site features do not explain catalysis, suggesting that the homodimer requires an additional factor as coGAP. Access to the catalytic center and an unusual switch I movement in the dimeric structure support this finding. Potential binding sites for interactions within the Toc translocon or with precursor proteins can be derived from the structures.

Policing Tic 'n' Toc, the doorway to chloroplasts

The organization of eukaryotic cells into different membrane-enclosed compartments requires an ordered and regulated system for targeting and translocating proteins synthesized in the cytosol across organellar membranes. Protein translocation through integral membrane proteinaceous complexes shares common principles in different organelles, whereas molecular mechanisms and energy requirements are diverse. Translocation into mitochondria and plastids requires most proteins to cross two membranes, and translocation must be regulated to accommodate environmental or metabolic changes. In the last decade, the first ideas were formulated about the regulation of protein translocation into chloroplasts, thereby laying the foundation for this field. Here, we describe recent models for the regulation of translocation by precursor protein phosphorylation, receptor dimerization, redox sensing and calcium signaling. We suggest how these mechanisms might fit within the regulatory framework for the entry of proteins into chloroplasts.