Arabidopsis genes encoding components of the chloroplastic protein import apparatus - PubMed (original) (raw)

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Arabidopsis genes encoding components of the chloroplastic protein import apparatus

D Jackson-Constan et al. Plant Physiol. 2001 Apr.

Abstract

The process of protein import into plastids has been studied extensively using isolated pea (Pisum sativum) chloroplasts. As a consequence, virtually all of the known components of the proteinaceous apparatus that mediates import were originally cloned from pea. With the recent completion of the Arabidopsis genome sequencing project, it is now possible to identify putative homologs of the import components in this species. Our analysis has revealed that Arabidopsis homologs with high sequence similarity exist for all of the pea import complex subunits, making Arabidopsis a valid model for further study of this system. Multiple homologs can be identified for over one-half of the components. In all but one case it is known that more than one of the putative isoforms for a particular subunit are expressed. Thus, it is possible that multiple types of import complexes are present within the same cell, each having a unique affinity for different chloroplastic precursor proteins, depending upon the exact mix of isoforms it contains. Sequence analysis of the putative Arabidopsis homologs for the chloroplast protein import apparatus has revealed many questions concerning subunit function and evolution. It should now be possible to use the genetic tools available in Arabidopsis, including the generation of knockout mutants and antisense technology, to address these questions and learn more about the molecular functions of each of the components during the import process.

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Figures

Figure 1

Figure 1

Current model describing the process of protein import into pea chloroplasts. Nuclear-encoded chloroplastic proteins are initially synthesized in the cytoplasm with a transit peptide (teal) that targets them to the plastid surface (a). In a process stimulated by GTP, the precursor protein associates with the components (blue) of the outer envelope membrane translocon (b). Hydrolysis of ATP in the cytoplasm and/or intermembrane space causes the precursor to interact with the components (green) of the inner membrane translocon as well (c). It is postulated that this step may be assisted by chaperones residing in the intermembrane space (purple). Hydrolysis of stromal ATP results in the complete translocation of the precursor protein into the chloroplast interior, where the transit peptide is removed (d). This final step is mediated at least in part by stromal factors (red). The numbers within the components of the outer and inner membrane translocons refer to the calculated molecular mass of each subunit. OM, Outer membrane; IM, inner membrane.

Figure 2

Figure 2

Multiple sequence alignment for the putative chloroplast-localized Arabidopsis Hsp70 isoforms. Shaded residues designate sequence identities between two or more of the proteins. The predicted transit peptide is indicated (>). The predicted cleavage site is based on sequence identity to a pea chloroplastic Hsp70 (accession no. L03299) and has not been experimentally verified. The alignment was created using the PileUp program from the Wisconsin package of sequence analysis tools (Genetics Computer Group, Madison, WI).

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