Functional Specialization amongst the Arabidopsis Toc159 Family of Chloroplast Protein Import Receptors[W]

Abstract

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.

The gateway to chloroplast: re-defining the function of chloroplast receptor proteins

“…These phenotypic defects, however, were restored to that of the wild-type in mature plants (Gutensohn et al , 2000 ). Similar to the ppi 2 mutant, the ppi 1 mutant renders a down-regulation of photosynthetic genes but the ' housekeeping ' genes are unaffected (Kubis et al , 2004 ). Isolated ppi 1 chloroplasts demonstrate an import defect of a representative photosynthetic protein in comparison to a non-photosynthetic protein (Kubis et al , 2004 ).…”

“…atTOC33 is expressed at very high levels in young, rapidly expanding photosynthetic tissues, whereas atTOC34 is expressed at low levels throughout development (Gutensohn et al , 2000 ;Kubis et al , 2004 ;Jarvis , 2008 ). Evidence for the functional specificity of these receptors to the TOC complex has emerged from the analysis of the atToc33 and atToc34 T DNA knockout mutants.…”

“…Similar to the ppi 2 mutant, the ppi 1 mutant renders a down-regulation of photosynthetic genes but the ' housekeeping ' genes are unaffected (Kubis et al , 2004 ). Isolated ppi 1 chloroplasts demonstrate an import defect of a representative photosynthetic protein in comparison to a non-photosynthetic protein (Kubis et al , 2004 ). The atToc34 knockout mutant ( ppi 3), on the other hand, shows normal development of green tissues but root growth is slightly impaired (Constan et al , 2004 ).…”

“…This did not appear to be the case for non-photosynthetic plastid proteins. This observation led to the proposal that proteins from this class are imported by other members of the large Toc GTPase family, namely atToc132 and atToc120 (Bauer et al , 2000 ;Kubis et al , 2004 ). These different receptors were assembled into structurally-distinct translocation complexes that exhibit unique preprotein binding properties that differ to that of atToc159 (Figure 2 ; Bauer et al , 2000 ;Ivanova et al , 2004 ;Kubis et al, 2004 ).…”

“…The Arabidopsis genome encodes two paralogs of Toc34 (atToc33 and atToc34); Gutensohn et al , 2000 ), and four paralogs of Toc159 (atToc159, atToc132, atToc120 and atToc90; Bauer et al , 2000 ;Hiltbrunner et al , 2004 ;Kubis et al , 2004 ) and Toc75 (atToc75-III, atToc75-IV, atToc75-I and atToc75V/ atOep80) (Eckart et al , 2002 ;Baldwin et al, 2005). The discovery of multiple arrays of isoforms in Arabidopsis promises to make the functional characterization of each individual component of the chloroplast import system challenging.…”