Chloroplast unfolded protein response, a new plastid stresssignaling pathway?

摘要

Aunique feature of the ATP-dependent ClpP protease of eukary-otic photosynthetic organisms is that itscatalytic subunit ClpP1 is encoded by thechloroplast genome. Attempts to inacti-vate this subunit through chloroplasttransformation have failed because it isessential for cell survival. To study thefunction of ClpP we have developed arepressible chloroplast gene expressionsystem inChlamydomonas reinhardtii.This system is based on the use of a chi-meric nuclear gene in which the vitamin-repressibleMetEpromoter andThi4riboswitch have been fused to the codingsequence ofNac2. Upon entry into thechloroplast the Nac2 protein specificallyinteracts with thepsbD5’UTR and isrequired for the proper processing/trans-lation of thepsbDmRNA. This propertycanbe conveyed to any chloroplastmRNA by replacing its 5’UTR with thatofpsbD. In this study we have chosenclpP1as plastid target gene and examinedthe cellular events induced upon deple-tion of ClpP through transcriptomic,proteomic, biochemical andelectronmicroscope analysis. Among the moststriking features, a massive increase inprotein abundance occurs for plastidchaperones, proteases and proteinsinvolved in membrane assembly/disas-sembly strongly suggesting the existenceof a chloroplast unfolded proteinresponse.