The Arabidopsisppi1Mutant Is Specifically Defective in the Expression, Chloroplast Import, and Accumulation of Photosynthetic Proteins[W]

Abstract

The import of nucleus-encoded proteins into chloroplasts is mediated by translocon complexes in the envelope membranes. A component of the translocon in the outer envelope membrane, Toc34, is encoded in Arabidopsis by two homologous genes, atTOC33 and atTOC34 . Whereas atTOC34 displays relatively uniform expression throughout development, atTOC33 is strongly upregulated in rapidly growing, photosynthetic tissues. To understand the reason for the existence of these two related genes, we characterized the atTOC33 knockout mutant ppi1 . Immunoblotting and proteomics revealed that components of the photosynthetic apparatus are deficient in ppi1 chloroplasts and that nonphotosynthetic chloroplast proteins are unchanged or enriched slightly. Furthermore, DNA array analysis of 3292 transcripts revealed that photosynthetic genes are moderately, but specifically, downregulated in ppi1 . Proteome differences in ppi1 could be correlated with protein import rates: ppi1 chloroplasts imported the ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit and 33-kD oxygen-evolving complex precursors at significantly reduced rates, but the import of a 50S ribosomal subunit precursor was largely unaffected. The ppi1 import defect occurred at the level of preprotein binding, which is consistent with a role for atToc33 during preprotein recognition. The data suggest that atToc33 is involved preferentially in the import of photosynthetic proteins and, by extension, that atToc34 is involved in the import of nonphotosynthetic chloroplast proteins.

Plastid Proteome Assembly without Toc159: Photosynthetic Protein Import and Accumulation of N-Acetylated Plastid Precursor Proteins

“…Of these genes, 1998 (8.2%) were significantly upregulated in the wild type and 2268 (9.3%) in ppi2 ( Figure 3A; see Supplemental Data Set 4 online). Our transcriptomics data confirmed the trend that photosynthetic proteins are expressed at lower levels in the import mutant as previously reported in published microarray, SAGE and RT-PCR data (Kubis et al, 2003;Kakizaki et al, 2009;Lee et al, 2009b). Notably, 84% of the downregulated genes reported in the SAGE data set were also found downregulated under our experimental conditions (see Supplemental Data Set 4 online).…”

“…The receptors are required for photosynthetic growth because their loss-of-function produces plants with a pale green or albino phenotype (Jarvis et al, 1998;Bauer et al, 2000). The analysis of Toc33 mutants (ppi1 for plastid protein import 1) suggested that this receptor may be preferentially involved in the import of photosynthetic proteins (Kubis et al, 2003). Similarly, the loss of Toc159 function (ppi2) results in albino plants that do not grow beyond the cotyledon stage on soil because the accumulation of photosynthetic proteins is significantly reduced (Bauer et al, 2000).…”

CIA2 Coordinately Up-Regulates Protein Import and Synthesis in Leaf Chloroplasts

“…The import efficiency of four photosynthesis-related proteins (RbcS, chlorophyll a/b-binding protein [Cab], and PORA and PORB [protochlorophyllide oxidoreductases A and B]) into ppi1 mutant chloroplasts is reduced compared to wild-type chloroplasts (Jarvis et al, 1998). Furthermore, the steady-state amount of RbcS has approximately 50% reduction in ppi1 chloroplasts (Kubis et al, 2003), revealing that deficiency of Toc33 Figure 7. Up-regulation of translocon and ribosomal genes by CIA2.…”

Members of the Toc159 Import Receptor Family Represent Distinct Pathways for Protein Targeting to Plastids

“…Two differentially expressed genes of the Toc34 family are present in Arabidopsis, atTOC33, and atTOC34 (Jarvis et al, 1998;Gutensohn et al, 2000), prompting the proposal that the proteins play specific roles during plastid development. atToc33 and atToc34 seem to exhibit some preferences for binding to transit peptides (Gutensohn et al, 2000;Jelic et al, 2003;Kubis et al, 2003), suggesting that they might be involved in the import of distinct preproteins. However, ectopic expression of atToc34 can rescue the pale phenotype of an atToc33 null mutant (Jarvis et al, 1998), ppi1, indicating that the GTPases play overlapping roles in plastid protein import.…”