Itzhaki H, Naveh L, Lindahl M, Cook M, Adam Z
Department of Agricultural Botany, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76100, Israel.
J Biol Chem. 1998 Mar 20;273(12):7094-8. doi: 10.1074/jbc.273.12.7094.
The proteases involved in proteolytic degradation in the thylakoid lumen are largely unknown. Western analysis with an antibody against the Escherichia coli periplasmic serine protease DegP suggested that pea chloroplasts contain a homologue of this protease. This homologue was peripherally bound to the luminal side of the thylakoid membrane and could only be removed by a combination of high salt and non-ionic detergent. Its level increased almost 2-fold in pea seedlings exposed to elevated temperature for 4 h, suggesting this protease's role in the chloroplast's heat response. Isolated thylakoid membranes containing the chloroplastic homologue of DegP degraded beta-casein, an in vitro substrate of the bacterial protease. This activity was partially inhibited by a serine protease inhibitor, suggesting that at least part of the casein-degrading activity in the thylakoid membrane is attributable to DegP. The existence of chloroplastic DegP was further supported by isolating a full-length Arabidopsis cDNA (designated AtDegP) encoding a protein that is 37% identical and 60% similar to the E. coli protease. The amino terminus of the deduced amino acid sequence contained a bipartite transit peptide, typical of proteins targeted to the thylakoid lumen, and the mature portion of the protein contained the highly conserved serine protease catalytic triad His-Asp-Ser. The possible physiological roles of chloroplastic DegP protease are discussed.
类囊体腔中参与蛋白水解降解的蛋白酶在很大程度上尚不清楚。用针对大肠杆菌周质丝氨酸蛋白酶DegP的抗体进行的蛋白质免疫印迹分析表明，豌豆叶绿体中含有这种蛋白酶的同源物。该同源物外周结合在类囊体膜的腔侧，只有通过高盐和非离子洗涤剂的组合才能去除。在暴露于高温4小时的豌豆幼苗中，其水平几乎增加了2倍，表明这种蛋白酶在叶绿体热响应中的作用。含有DegP叶绿体同源物的分离类囊体膜降解了β-酪蛋白，β-酪蛋白是细菌蛋白酶的体外底物。这种活性被丝氨酸蛋白酶抑制剂部分抑制，表明类囊体膜中至少部分酪蛋白降解活性归因于DegP。通过分离编码与大肠杆菌蛋白酶有37%同一性和60%相似性的蛋白质的全长拟南芥cDNA（命名为AtDegP），进一步支持了叶绿体DegP的存在。推导的氨基酸序列的氨基末端包含一个双功能转运肽，这是靶向类囊体腔的蛋白质的典型特征，并且该蛋白质的成熟部分包含高度保守的丝氨酸蛋白酶催化三联体His-Asp-Ser。本文讨论了叶绿体DegP蛋白酶可能的生理作用。
