The outer membrane Omp85‐like protein P39 influences metabolic homeostasis in mature Arabidopsis thaliana

Abstract

The Omp85 proteins form a large membrane protein family in bacteria and eukaryotes. Omp85 proteins are composed of a C-terminal β-barrel-shaped membrane domain and one or more N-terminal polypeptide transport-associated (POTRA) domains. However, Arabidopsis thaliana contains two genes coding for Omp85 proteins without a POTRA domain. One gene is designated P39, according to the molecular weight of the encoded protein. The protein is targeted to plastids and it was established that p39 has electrophysiological properties similar to other Omp85 family members, particularly to that designated as Toc75V/Oep80. We analysed expression of the gene and characterised two T-DNA insertion mutants, focusing on alterations in photosynthetic activity, plastid ultrastructure, global expression profile and metabolome. We observed pronounced expression of P39, especially in veins. Mutants of P39 show growth aberrations, reduced photosynthetic activity and changes in plastid ultrastructure, particularly in the leaf tip. Further, they display global alteration of gene expression and metabolite content in leaves of mature plants. We conclude that the function of the plastid-localised and vein-specific Omp85 family protein p39 is important, but not essential, for maintenance of metabolic homeostasis of full-grown A. thaliana plants. Further, the function of p39 in veins influences the functionality of other plant tissues. The link connecting p39 function with metabolic regulation in mature A. thaliana is discussed.

Ubiquitin-dependent chloroplast-associated protein degradation in plants

“…The sp1-1, sp1-3, ppi1, tic40-4, hsp93-V-1, ppi2-3 (fts1), toc75-III-3 (mar1) mutants, and the 35S promoter-driven SP1 overexpressor (SP1-OX) transgenic line, have all been described previously (7,12,16,19,34,35). The sp2-4 (SALK_137135) mutant was obtained from the Salk Institute Genomic Analysis Laboratory, and confirmed by PCR and RT-PCR analysis, as described previously (36); this mutant was phenotypically similar to the three chemically-induced sp2 alleles identified in this study, and unlike a previously-described T-DNA mutant (37). When making double and triple mutants, the sp1-3 and sp2-4 alleles were employed (because their mutations are easier to detect by PCR); double mutants were selected and verified by phenotype analysis and by using PCR-based genotyping (table S1).…”

Toc75‐V/OEP80 is processed during translocation into chloroplasts, and the membrane‐embedded form exposes its POTRA domain to the intermembrane space

“…Like Toc75-III [10], Toc75-V is essential for plant development [13][14][15]. Two additional genes with similarity to Toc75-V have been identified as well, namely P39 and P36 [16][17][18][19][20]. Interestingly, these two proteins do not contain N-terminal POTRA domains and mutants of these two genes are viable [16,18].…”

“…Two additional genes with similarity to Toc75‐V have been identified as well, namely P39 and P36 . Interestingly, these two proteins do not contain N‐terminal POTRA domains and mutants of these two genes are viable . Moreover, P39 and P36 originated from a recent genome duplication of A. thaliana and only one gene is found in most of the other plant species examined .…”