阿拉伯芥葉綠體蛋白J25的功能與特性分析

Abstract

70 kD的熱休克蛋白 (heat shock protein 70 kD, Hsp70) 家族是細胞中普遍存在的伴護子 (chaperone)，參與蛋白質轉位 (protein translocation)、摺疊 (folding) 及降解 (degradation) 的過程。植物葉綠體內的cpHsc70最近被發現參與在前驅蛋白 (precursor protein) 輸入葉綠體的轉位過程。因為Hsp70在細胞中參與的功能太多，需要J蛋白來協助Hsp70的功能專一性，但在葉綠體內的J蛋白至少有19個，所以我們實驗室欲找出幫助cpHsc70進行轉位作用的J蛋白。本篇文章裡研究的J25蛋白被預測具有穿膜區塊 (transmembrane domain)，在本篇論文中，我發現J25不只位在類囊體膜 (thylakoid membrane)上，還有一小部分位在葉綠體內膜 (inner envelope membrane) 上。然而，從我的研究得知，j25突變株並不會造成蛋白質輸入葉綠體的過程受損。而此時，我們意外發現，在白光及紅光下生長的j25-1突變植株比起野生型有較長下胚軸 (hypocotyls) 的外表型，且j25與phyB (phytochrome B是最主要的紅光受器) 的雙突變株會有比兩個單突變株更長的下胚軸。另外，j25/phyB雙突變株種子的萌芽率也比兩個單突變株來的差。藉由j25突變株的特性分析，得知J25位在與紅光受器phyB不同的光訊息傳遞路徑上，讓我們得以將葉綠體的熱休克蛋白cpHsc70藉由J25與光傳導的訊息路徑做可能的連結。

The 70 kD heat shock proteins (Hsp70s) are involved in protein translocation, folding and degradation. We have recently shown that chloroplast Hsp70s, cpHsc70s, are involved in protein import into chloroplasts. Since the J protein cochaperones are drivers that recruit Hsp70 for specific functions, we intend to identify the J protein assisting cpHsc70s in chloroplast protein import. There are more than one hundred J proteins in Arabidopsis, and at least 19 of them are localized in chloroplasts. Among them, J25 is predicted to localize in membranes. I found that J25 has a dual localization: the majority of J25 is localized in the thylakoid membrane and a small portion is localized in the inner envelope membrane. However, the j25 mutants show no defect in the import of a model chloroplast precursor protein. Nonetheless, I found another interesting phenotype of the j25 mutants: one of the alleles has long hypocotyls when grown under white and red light and double mutants of both alleles with a phyB (phytochrome B, the major red light sensor of plants) mutant have an additive long hypocotyl phenotype under white light and red light. In addition, the j25/phyB double mutants have an additive defect in seeds germination. These data suggest that J25 is involved in light sensing and that J25 and phyB are involved in independent red light signaling pathways. Characterizations of J25 provide a possible link between chloroplast Hsp70 and light-regulated signal transduction.