Functional specialization amongst the Arabidopsis Toc159 family of chloroplast protein import receptors

Plant Cell. 2004 Aug;16(8):2059-77. doi: 10.1105/tpc.104.023309. Epub 2004 Jul 23.


The initial stages of preprotein import into chloroplasts are mediated by the receptor GTPase Toc159. In Arabidopsis thaliana, Toc159 is encoded by a small gene family: atTOC159, atTOC132, atTOC120, and atTOC90. Phylogenetic analysis suggested that at least two distinct Toc159 subtypes, characterized by atToc159 and atToc132/atToc120, exist in plants. atTOC159 was strongly expressed in young, photosynthetic tissues, whereas atTOC132 and atTOC120 were expressed at a uniformly low level and so were relatively prominent in nonphotosynthetic tissues. Based on the albino phenotype of its knockout mutant, atToc159 was previously proposed to be a receptor with specificity for photosynthetic preproteins. To elucidate the roles of the other isoforms, we characterized Arabidopsis knockout mutants for each one. None of the single mutants had strong visible phenotypes, but toc132 toc120 double homozygotes appeared similar to toc159, indicating redundancy between atToc132 and atToc120. Transgenic complementation studies confirmed this redundancy but revealed little functional overlap between atToc132/atToc120 and atToc159 or atToc90. Unlike toc159, toc132 toc120 caused structural abnormalities in root plastids. Furthermore, when proteomics and transcriptomics were used to compare toc132 with ppi1 (a receptor mutant that is specifically defective in the expression, import, and accumulation of photosynthetic proteins), major differences were observed, suggesting that atToc132 (and atToc120) has specificity for nonphotosynthetic proteins. When both atToc159 and the major isoform of the other subtype, atToc132, were absent, an embryo-lethal phenotype resulted, demonstrating the essential role of Toc159 in the import mechanism.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Arabidopsis / anatomy & histology
  • Arabidopsis / genetics
  • Arabidopsis / metabolism*
  • Arabidopsis Proteins / classification
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism
  • Chloroplasts / genetics
  • Chloroplasts / metabolism*
  • GTP Phosphohydrolases / classification
  • GTP Phosphohydrolases / genetics
  • GTP Phosphohydrolases / metabolism*
  • Gene Expression Profiling
  • Genetic Complementation Test
  • Genotype
  • Membrane Proteins / classification
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism*
  • Molecular Sequence Data
  • Multigene Family
  • Mutation
  • Oligonucleotide Array Sequence Analysis
  • Phenotype
  • Phylogeny
  • Plants, Genetically Modified
  • Plastids / metabolism
  • Plastids / ultrastructure
  • Protein Isoforms / genetics
  • Protein Isoforms / metabolism*
  • Protein Transport / physiology
  • Proteomics
  • Receptors, Cytoplasmic and Nuclear / genetics
  • Receptors, Cytoplasmic and Nuclear / metabolism*
  • Seeds / cytology
  • Seeds / genetics


  • AT2G16640 protein, Arabidopsis
  • Arabidopsis Proteins
  • Carrier Proteins
  • Membrane Proteins
  • Ppi1 protein, Arabidopsis
  • Protein Isoforms
  • Receptors, Cytoplasmic and Nuclear
  • TOC159 protein, Arabidopsis
  • GTP Phosphohydrolases

Associated data

  • GENBANK/AV523359
  • GENBANK/AV548084
  • GENBANK/P01112
  • GENBANK/W43632
  • RefSeq/NM_122037