OEP80, an essential protein paralogous to the chloroplast protein translocation channel Toc75, exists as a 70-kD protein in the Arabidopsis thaliana chloroplast outer envelope

Toc75 and OEP80 are paralogous proteins found in the Viridiplantae lineages, and appear to have evolved from a protein in the outer membrane of an ancient cyanobacterium. Toc75 is known to act as a protein translocation channel at the outer membrane of the chloroplast envelope, whereas the exact function of OEP80 is not understood. In Arabidopsis thaliana, each protein is encoded by a single gene, and both are essential for plant viability from embryonic stages onward. Sequence annotation and immunoblotting data with an antibody against its internal sequence (alpha OEP80(325-337)) indicated that the molecular weight of OEP80 is ca. 80 kD. Here we present multiple data to show that the size of A. thaliana OEP80 is smaller than previously estimated. First, we prepared the antibody against a recombinant protein consisting of annotated full-length A. thaliana OEP80 with an N-terminal hexahistidine tag (alpha OEP80(1-732)). This antibody recognized a 70-kD protein in the A. thaliana chloroplast membrane fraction which migrated faster than the His-tagged antigen and the protein recognized by the alpha OEP80(325-337) antibody on SDS-PAGE. Immunoprecipitation followed by LC-MS/MS analysis confirmed that the 70-kD protein was encoded by the OEP80 cDNA. Next, we performed a genetic complementation assay using embryo-lethal oep80-null plants and constructs encoding OEP80 and its variants. The results revealed that the nucleotide sequence encoding the 52 N-terminal amino acids was not required for functional expression of OEP80 and accumulation of the 70-kD protein. The data also indicated that an additional C-terminal T7 tag remained intact without disrupting the functionality of OEP80, and was not exposed to the cytoplasmic surface of the chloroplast envelope. Finally, OEP80-T7 and Toc75 showed distinct migration patterns on blue native-PAGE. This study provides molecular tools to investigate the function of OEP80, and also calls for caution in using an anti-peptide antibody.