Tranel P J, Froehlich J, Goyal A, Keegstra K
MSU-DOE-Plant Research Laboratory, Michigan State University, East Lansing 48824-1312, USA.
EMBO J. 1995 Jun 1;14(11):2436-46. doi: 10.1002/j.1460-2075.1995.tb07241.x.
A chloroplastic outer envelope membrane protein of 75 kDa (OEP75) was identified previously as a component of the protein import machinery. Here we provide additional evidence that OEP75 is a component of protein import, present the isolation of a cDNA clone encoding this protein, briefly describe its developmental expression and tissue specificity, and characterize its insertion into the outer envelope membrane. OEP75 was synthesized as a higher molecular weight precursor (prOEP75) which bound to isolated chloroplasts in an in vitro import assay and subsequently was processed to the mature form (mOEP75). During this import assay, two proteins intermediate in size between prOEP75 and mOEP75 were detected. One of these intermediates was also detected in chloroplast envelopes isolated from young pea leaves. Binding and processing of prOEP75 required ATP and one or more surface-exposed proteinaceous components, and was competed by prSSU, a stromal-targeted protein. We propose that the N-terminus of the prOEP75 transit peptide acts as a stromal-targeting domain and a central, hydrophobic region of this transit peptide acts as a stop-transfer domain. A complex route of insertion and processing of prOEP75 may exist to ensure high fidelity targeting of this import component.
一种75 kDa的叶绿体外膜蛋白（OEP75）先前被鉴定为蛋白质输入机制的一个组成部分。在此，我们提供了更多证据证明OEP75是蛋白质输入的一个组成部分，展示了编码该蛋白质的cDNA克隆的分离，简要描述了其发育表达和组织特异性，并对其插入外膜的过程进行了表征。OEP75最初以较高分子量的前体形式（prOEP75）合成，在体外输入试验中它与分离的叶绿体结合，随后被加工成成熟形式（mOEP75）。在这个输入试验过程中，检测到了两种大小介于prOEP75和mOEP75之间的中间蛋白。其中一种中间蛋白在从嫩豌豆叶中分离的叶绿体被膜中也被检测到。prOEP75的结合和加工需要ATP以及一种或多种表面暴露的蛋白质成分，并且会受到基质靶向蛋白prSSU的竞争。我们提出，prOEP75转运肽的N端作为基质靶向结构域，而该转运肽的一个中央疏水区域作为停止转移结构域。可能存在一条复杂的prOEP75插入和加工途径，以确保该输入成分的高保真靶向。
