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Evolution and targeting of Omp85 homologs in the chloroplast outer envelope membrane.

Day PM, Potter D, Inoue K - Front Plant Sci (2014)

Bottom Line: Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent.The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80.Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

View Article: PubMed Central - PubMed

Affiliation: Department of Plant Sciences, University of California at Davis Davis, CA, USA.

ABSTRACT
Translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75) is the core component of the chloroplast protein import machinery. It belongs to the Omp85 family whose members exist in various Gram-negative bacteria, mitochondria, and chloroplasts of eukaryotes. Chloroplasts of Viridiplantae contain another Omp85 homolog called outer envelope protein 80 (OEP80), whose exact function is unknown. In addition, the Arabidopsis thaliana genome encodes truncated forms of Toc75 and OEP80. Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent. The bipartite targeting sequence-dependent sorting of Toc75 has been demonstrated but the targeting mechanisms of other chloroplast Omp85 homologs remain largely unexplored. This study was aimed to address these unresolved issues in order to further our understanding of chloroplast evolution. Sequence alignments and recently determined structures of bacterial Omp85 homologs were used to predict structures of chloroplast Omp85 homologs. The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80. Phylogenetic analyses using Omp85 homologs from various cyanobacteria and chloroplasts provided strong support for the grouping of Toc75 and OEP80 sister to cyanoOmp85. However, this support was diminished when the analysis included Omp85 homologs from other bacteria and mitochondria. Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

No MeSH data available.


Molecular architecture of the Omp85 family proteins. Omp85 homologs are comprised of an N-terminal soluble portion containing a variable number of POTRA domains (orange) followed by a C-terminal transmembrane β-barrel (green). The POTRA domains 1, 2, and 3 of the chloroplast and cyanobacteria homologs are indicated as P1, P2, and P3, respectively. Unlike the others, OEP80tr and Toc75-IV do not contain any full POTRA domains although they both contain sequences that align well with the last β-strand of P3 in their relatives. Within the β-barrel that is made of 16 transmembrane β-strands, strands 11 and 12 are separated by the 6th extracellular loop (blue) which contains a sequence highly conserved between all Omp85 homologs. The homologs from bacteria contain a signal peptide (turquoise) that is required for protein export from the cytoplasm to the periplasm. Toc75 contains a unique bipartite targeting signal (gray) at its N terminus and an apparent unique insertion at the beginning of its second POTRA domain. Cyanobacterial Omp85 homologs and TpsB contain a Pro-rich region (pink) N terminus to the first POTRA domain. In addition, TpsB contains an α-helix (brown) N terminus to the Pro-rich region. The domain lengths are approximately to scale.
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Figure 1: Molecular architecture of the Omp85 family proteins. Omp85 homologs are comprised of an N-terminal soluble portion containing a variable number of POTRA domains (orange) followed by a C-terminal transmembrane β-barrel (green). The POTRA domains 1, 2, and 3 of the chloroplast and cyanobacteria homologs are indicated as P1, P2, and P3, respectively. Unlike the others, OEP80tr and Toc75-IV do not contain any full POTRA domains although they both contain sequences that align well with the last β-strand of P3 in their relatives. Within the β-barrel that is made of 16 transmembrane β-strands, strands 11 and 12 are separated by the 6th extracellular loop (blue) which contains a sequence highly conserved between all Omp85 homologs. The homologs from bacteria contain a signal peptide (turquoise) that is required for protein export from the cytoplasm to the periplasm. Toc75 contains a unique bipartite targeting signal (gray) at its N terminus and an apparent unique insertion at the beginning of its second POTRA domain. Cyanobacterial Omp85 homologs and TpsB contain a Pro-rich region (pink) N terminus to the first POTRA domain. In addition, TpsB contains an α-helix (brown) N terminus to the Pro-rich region. The domain lengths are approximately to scale.

Mentions: Chloroplasts are derived from an endosymbiotic relationship between an ancestral cyanobacterium and a mitochondriate eukaryote which occurred around 1 billion years ago (Shih and Matzke, 2013). One piece of evidence to support a common ancestry of Gram-negative bacteria, mitochondria and chloroplasts is the presence of β-barrel proteins in their outer membranes (Inoue, 2007). Among β-barrel membrane proteins are the homologs of outer membrane protein 85 (Omp85) which appear to be present in all Gram-negative bacteria, mitochondria and chloroplasts (Voulhoux et al., 2003; Gentle et al., 2004, 2005; Voulhoux and Tommassen, 2004). A canonical member of the Omp85 family is comprised of a soluble N terminus which contains a variable number of polypeptide translocation associated (POTRA) domains, each of which usually consists of 70–90 residues, followed by a C-terminal transmembrane β-barrel (Sanchez-Pulido et al., 2003) (Figure 1). Results of various studies indicate that the POTRA domains are involved in association with other proteins while the β-barrel acts as an integral membrane anchor and may provide a hydrophilic pore to accommodate substrate proteins (Knowles et al., 2009; Kim et al., 2012; Simmerman et al., 2014).


Evolution and targeting of Omp85 homologs in the chloroplast outer envelope membrane.

Day PM, Potter D, Inoue K - Front Plant Sci (2014)

Molecular architecture of the Omp85 family proteins. Omp85 homologs are comprised of an N-terminal soluble portion containing a variable number of POTRA domains (orange) followed by a C-terminal transmembrane β-barrel (green). The POTRA domains 1, 2, and 3 of the chloroplast and cyanobacteria homologs are indicated as P1, P2, and P3, respectively. Unlike the others, OEP80tr and Toc75-IV do not contain any full POTRA domains although they both contain sequences that align well with the last β-strand of P3 in their relatives. Within the β-barrel that is made of 16 transmembrane β-strands, strands 11 and 12 are separated by the 6th extracellular loop (blue) which contains a sequence highly conserved between all Omp85 homologs. The homologs from bacteria contain a signal peptide (turquoise) that is required for protein export from the cytoplasm to the periplasm. Toc75 contains a unique bipartite targeting signal (gray) at its N terminus and an apparent unique insertion at the beginning of its second POTRA domain. Cyanobacterial Omp85 homologs and TpsB contain a Pro-rich region (pink) N terminus to the first POTRA domain. In addition, TpsB contains an α-helix (brown) N terminus to the Pro-rich region. The domain lengths are approximately to scale.
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Related In: Results  -  Collection

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Figure 1: Molecular architecture of the Omp85 family proteins. Omp85 homologs are comprised of an N-terminal soluble portion containing a variable number of POTRA domains (orange) followed by a C-terminal transmembrane β-barrel (green). The POTRA domains 1, 2, and 3 of the chloroplast and cyanobacteria homologs are indicated as P1, P2, and P3, respectively. Unlike the others, OEP80tr and Toc75-IV do not contain any full POTRA domains although they both contain sequences that align well with the last β-strand of P3 in their relatives. Within the β-barrel that is made of 16 transmembrane β-strands, strands 11 and 12 are separated by the 6th extracellular loop (blue) which contains a sequence highly conserved between all Omp85 homologs. The homologs from bacteria contain a signal peptide (turquoise) that is required for protein export from the cytoplasm to the periplasm. Toc75 contains a unique bipartite targeting signal (gray) at its N terminus and an apparent unique insertion at the beginning of its second POTRA domain. Cyanobacterial Omp85 homologs and TpsB contain a Pro-rich region (pink) N terminus to the first POTRA domain. In addition, TpsB contains an α-helix (brown) N terminus to the Pro-rich region. The domain lengths are approximately to scale.
Mentions: Chloroplasts are derived from an endosymbiotic relationship between an ancestral cyanobacterium and a mitochondriate eukaryote which occurred around 1 billion years ago (Shih and Matzke, 2013). One piece of evidence to support a common ancestry of Gram-negative bacteria, mitochondria and chloroplasts is the presence of β-barrel proteins in their outer membranes (Inoue, 2007). Among β-barrel membrane proteins are the homologs of outer membrane protein 85 (Omp85) which appear to be present in all Gram-negative bacteria, mitochondria and chloroplasts (Voulhoux et al., 2003; Gentle et al., 2004, 2005; Voulhoux and Tommassen, 2004). A canonical member of the Omp85 family is comprised of a soluble N terminus which contains a variable number of polypeptide translocation associated (POTRA) domains, each of which usually consists of 70–90 residues, followed by a C-terminal transmembrane β-barrel (Sanchez-Pulido et al., 2003) (Figure 1). Results of various studies indicate that the POTRA domains are involved in association with other proteins while the β-barrel acts as an integral membrane anchor and may provide a hydrophilic pore to accommodate substrate proteins (Knowles et al., 2009; Kim et al., 2012; Simmerman et al., 2014).

Bottom Line: Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent.The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80.Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

View Article: PubMed Central - PubMed

Affiliation: Department of Plant Sciences, University of California at Davis Davis, CA, USA.

ABSTRACT
Translocon at the outer-envelope-membrane of chloroplasts 75 (Toc75) is the core component of the chloroplast protein import machinery. It belongs to the Omp85 family whose members exist in various Gram-negative bacteria, mitochondria, and chloroplasts of eukaryotes. Chloroplasts of Viridiplantae contain another Omp85 homolog called outer envelope protein 80 (OEP80), whose exact function is unknown. In addition, the Arabidopsis thaliana genome encodes truncated forms of Toc75 and OEP80. Multiple studies have shown a common origin of the Omp85 homologs of cyanobacteria and chloroplasts but their results about evolutionary relationships among cyanobacterial Omp85 (cyanoOmp85), Toc75, and OEP80 are inconsistent. The bipartite targeting sequence-dependent sorting of Toc75 has been demonstrated but the targeting mechanisms of other chloroplast Omp85 homologs remain largely unexplored. This study was aimed to address these unresolved issues in order to further our understanding of chloroplast evolution. Sequence alignments and recently determined structures of bacterial Omp85 homologs were used to predict structures of chloroplast Omp85 homologs. The results enabled us to identify amino acid residues that may indicate functional divergence of Toc75 from cyanoOmp85 and OEP80. Phylogenetic analyses using Omp85 homologs from various cyanobacteria and chloroplasts provided strong support for the grouping of Toc75 and OEP80 sister to cyanoOmp85. However, this support was diminished when the analysis included Omp85 homologs from other bacteria and mitochondria. Finally, results of import assays using isolated chloroplasts support outer membrane localization of OEP80tr and indicate that OEP80 may carry a cleavable targeting sequence.

No MeSH data available.