Rhomboid proteins in the chloroplast envelope affect the level of allene oxide synthase in Arabidopsis thaliana

Rhomboids are intra‐membrane serine proteases whose sequences are found in nearly all organisms. They are involved in a variety of biological functions in both eukaryotes and prokaryotes. Localization assays revealed that two Arabidopsis thaliana rhomboid‐like proteases (AtRBL), AtRBL8 and AtRBL9, are targeted to the chloroplast. Using transgenic plants expressing epitope‐tagged AtRBL9, we localized AtRBL9 to the chloroplast inner envelope membrane, with both its N‐ and C‐termini facing the stroma. Mass spectrometry analyses confirmed this localization, and suggested that this is also the case for AtRBL8. Both are proteins of very low abundance. The results of size‐exclusion chromatography implied that AtRBL9 forms homo‐oligomers. In search of a putative function, a comparative proteomic analysis was performed on wild‐type and double‐knockout plants, lacking both AtRBL8 and AtRBL9, using the iTRAQ method. Of 180 envelope proteins, the level of only a few was either increased or decreased in the mutant line. One of the latter, allene oxide synthase, is involved in jasmonic acid biosynthesis. This observation provides an explanation for the recently reported aberration in flower morphology that is associated with the loss of AtRBL8.