Characterization of the Arabidopsis PPI1 Mutant

Abstract

Genetic screens for loci involved in the positive regulation of LHCP gene expression have identified a series of Arabidopsis mutants displaying altered plastid development. One such mutant, ppi1 (plastid protein import), has a pronounced pale or yellow-green phenotype throughout its life-cycle, although mature leaves frequently appear darker green than younger tissues. Molecular analyses of the ppi1 locus revealed the presence of a T-DNA insertion within the second intron of a gene for a protein with similarity to GTP-binding proteins of the plastidic outer envelope membrane. Specifically, the PPI1 gene encodes a protein of 33 kD which is highly homologous with Toc34 (translocon at the outer envelope membrane of chloroplasts, 34 kD), a protein involved in the translocation of proteins into the plastid. Import experiments using isolated chloroplasts demonstrated that the new protein, referred to as Toc33, inserts into the outer envelope membrane in a manner similar to Toc34. Given (1)that the majority of proteins entering the chloroplast do so through the general import apparatus (of which Toc34 is a component), (2) that the ppi1 mutation is null, and (3)that the ppi1 mutant phenotype is not severe, we believe that Toc33 is involved in the general import process as a redundant translocon component which acts interchangeably with Toc34. Consistently, complementation of the ppi1 genetic defect by over-expression of Toc34 demonstrated the functional similarity of the two homologs. Detailed characterization of ppi1 plants has provided clear evidence for the perturbation of plastid protein import in vivo. Our data also suggest that the expression pattern of Toc33 may be responsible, at least in part, for developmental changes in the import capabilities of plastids.