The Arabidopsis ppi1 mutant is specifically defective in the expression, chloroplast import, and accumulation of photosynthetic proteins.
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.
Kubis
Sybille
S
Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom.
Baldwin
Amy
A
Patel
Ramesh
R
Razzaq
Azam
A
Dupree
Paul
P
Lilley
Kathryn
K
Kurth
Joachim
J
Leister
Dario
D
Jarvis
Paul
P
D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism*; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D002736:Chloroplasts / Q000235:genetics / Q000378:metabolism; D015536:Down-Regulation; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010788:Photosynthesis / Q000235:genetics; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018749:RNA, Plant / Q000235:genetics / Q000378:metabolism
D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D008565:Membrane Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins; C469717:Ppi1 protein, Arabidopsis; D018749:RNA, Plant; C491798:TOC34 protein, Arabidopsis; C114786:Toc33 protein, Arabidopsis; C112475:Toc34 protein, plant