The Chloroplast Endoplasmic Reticulum: Structure, Function, and Evolutionary Significance

Gibbs, Sarah P.

doi:10.1016/s0074-7696(08)61194-8

Cited by 147 publications

(107 citation statements)

References 181 publications

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“…In chromophytic algae ribosomes have been observed to be attached to the cytosolic side of the outermost plastid membrane, and the outer two membranes are continuous with the nuclear envelope and cytoplasmic ER; therefore the outer membranes have been referred to as "chloroplast" ER (27). It has been proposed earlier that protein targeting into the plastids might involve a passage through rough ER membranes (28).…”

Section: Discussionmentioning

confidence: 99%

Protein Transport into “Complex” Diatom Plastids Utilizes Two Different Targeting Signals

Lang

Apt²,

Kroth

1998

Journal of Biological Chemistry

103

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The plastids found in diatoms and other chromophytic algae are completely enclosed by four membranes in contrast to chloroplasts of higher plants, which are surrounded by only two membranes. The bipartite targeting sequence of diatom nuclear-encoded plastid proteins contains an endoplasmic reticulum signal sequence and, based on sequence comparison, a transit peptide-like domain similar to that which targets proteins into the plastids of higher plants. By performing heterologous import experiments using the precursor of the ␥ subunit of the chloroplast ATPase from the diatom Odontella sinensis we were able to show that protein import into diatom plastids is at least a two-step event. We demonstrate that the first step involves co-translational transport through endoplasmic reticulum membranes and that there is an additional targeting step which is similar to the import of precursor proteins into chloroplasts of higher plants and green algae indicating that the transit peptide-like domain of the diatom precursor is functionally equivalent to the respective targeting signal of higher plants. Our results suggest that the transit peptide depending targeting mechanism in plastids has apparently remained relatively unchanged over the course of evolution, with only the peptidase cleavage site significantly modified.

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Section: Discussionmentioning

confidence: 99%

Protein Transport into “Complex” Diatom Plastids Utilizes Two Different Targeting Signals

Lang

Apt²,

Kroth

1998

Journal of Biological Chemistry

103

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“…In cryptophytes and diatoms there are two additional membranes around the plastids, the outermost being studded with ribosomes and continuous with the endoplasmic reticulum (ER) (Gibbs 1981). The plastid genomes of the diatom Phaeodactylum tricornutum and the cryptophyte Guillardia theta contain only 162 and 177 genes (Douglas and Penny 1999;Oudot-Le Secq et al 2007), however, a plastid proteome size similar to that of higher plants must be assumed because photosynthesis is a rather complex process.…”

Section: Introductionmentioning

confidence: 99%

Protein targeting into complex diatom plastids: functional characterisation of a specific targeting motif

et al. 2007

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Plastids of diatoms and related algae evolved by secondary endocytobiosis, the uptake of a eukaryotic alga into a eukaryotic host cell and its subsequent reduction into an organelle. As a result diatom plastids are surrounded by four membranes. Protein targeting of nucleus encoded plastid proteins across these membranes depends on N-terminal bipartite presequences consisting of a signal and a transit peptide-like domain. Diatoms and cryptophytes share a conserved amino acid motif of unknown function at the cleavage site of the signal peptides (ASA-FAP), which is particularly important for successful plastid targeting. Screening genomic databases we found that in rare cases the very conserved phenylalanine within the motif may be replaced by tryptophan, tyrosine or leucine. To test such unusual presequences for functionality and to better understand the role of the motif and putative receptor proteins involved in targeting, we constructed presequence:GFP fusion proteins with or without modifications of the ''ASAFAP''-motif and expressed them in the diatom Phaeodactylum tricornutum. In this comprehensive mutational analysis we found that only the aromatic amino acids phenylalanine, tryptophan, tyrosine and the bulky amino acid leucine at the +1 position of the predicted signal peptidase cleavage site allow plastid import, as expected from the sequence comparison of native plastid targeting presequences of P. tricornutum and the cryptophyte Guillardia theta. Deletions within the signal peptide domains also impaired plastid import, showing that the presence of F at the N-terminus of the transit peptide together with a cleavable signal peptide is crucial for plastid import.

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“…A third unique membrane completely surrounds the plastid envelope and is thought to represent the former plasma membrane of the endosymbiont, while the outermost membrane may have evolved from the vacuolar/plasma membrane of the host organism. This outermost membrane has been observed to be continuous with the endoplasmic reticulum (Gibbs, 1981;Ishida et al, 2000). The portion of this ER-like membrane directly adjacent to plastids (and which appears to completely encase the plastid) is commonly referred to as the chloroplast ER or CER (Bouck, 1965).…”

mentioning

confidence: 99%

In vivo characterization of diatom multipartite plastid targeting signals

Apt¹,

Zaslavkaia²,

Lippmeier³

et al. 2002

Journal of Cell Science

142

130

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Plastids of diatoms and related algae are delineated by four membranes: the outermost membrane (CER) is continuous with the endoplasmic reticulum while the inner two membranes are homologous to plastid envelope membranes of vascular plants and green algae. Proteins are transported into these plastids by pre-sequences that have two recognizable domains. To characterize targeting of polypeptides within diatom cells, we generated constructs encoding green fluorecent protein (GFP) fused to leader sequences. A fusion of GFP to the pre-sequence of BiP [an endoplasmic reticulum (ER)-localized chaperone]resulted in accumulation of GFP within the ER; a construct encoding the pre-sequence of a plastid protein fused to GFP was directed into the plastids. Additional constructs demonstrated that the N-terminal region of the bipartite plastid targeting pre-sequence was necessary for transport of polypeptides to the lumen of the ER, while the C-terminal region was shown to enable the proteins to traverse the plastid double envelope membrane. Our data strongly support the hypothesis of a multi-step plastid targeting process in chromophytic algae and raises questions about the continuity of the ER and CER and the function of the latter in polypeptide trafficking.

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The Chloroplast Endoplasmic Reticulum: Structure, Function, and Evolutionary Significance

Cited by 147 publications

References 181 publications

Protein Transport into “Complex” Diatom Plastids Utilizes Two Different Targeting Signals

Protein Transport into “Complex” Diatom Plastids Utilizes Two Different Targeting Signals

Protein targeting into complex diatom plastids: functional characterisation of a specific targeting motif

In vivo characterization of diatom multipartite plastid targeting signals

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