16.16: 蛋白质转运到基质

叶绿体是具有外膜、内膜和类囊体膜的三重膜结构，每个结构都包含不同的代谢物转运蛋白、膜转位蛋白和酶。对这些蛋白质进行适当的分选并将其转移到正确的膜系统中对于叶绿体功能至关重要。

称为叶绿体外膜转位子或 TOC 复合体的蛋白质复合体，以及叶绿体内膜或 TIC 复合体的转位子介导叶绿体蛋白的正确排序。新合成的靶向基质的前体结合胞质伴侣 Hsp90 和 Hsp70-Hsp90 组织蛋白 （HOP）。Hsp90 和 HOP 利用 ATP 水解的能量将前驱体保持在未折叠状态。N 端可切割的 13 至 146 个氨基酸长转运肽序列和不可切割的内部信号序列有助于前体易位到正确的叶绿体亚区室。转运肽被两种同源 GTP 酶 TOC159 和 TOC34 识别。TOC159 是转运肽的初始受体，而 TOC34 促进 GTP 水解，帮助前体通过 TOC 通道进入。

然后，前体通过 TIC 复合物穿过内膜运输。足够的 ATP 浓度加上适当的温度有助于前体穿过叶绿体外膜和内叶绿体膜转移到基质中。TIC40 还刺激 Hsp93 利用 ATP 水解的能量并将前体完全拉入基质中。基质加工肽酶在加工前体释放到基质中并被 Cpn60 折叠形成活性蛋白时裂解转运信号。Hsp93 经历 ADP 到 ATP 的交换，并在连续的导入周期中提供帮助。

大多数叶绿体蛋白在胞质溶胶中合成并作为未折叠的前体易位。

这些前体在 N 端包含一个可切割的传输信号和一个将蛋白质靶向不同亚区室的内部导入信号。

胞质伴侣与传输信号相互作用，并将前体引导至叶绿体外膜。

嵌入叶绿体膜中的多聚体复合物称为外叶绿体膜的转位酶或 TOC 复合物，而内叶绿体膜或 TIC 复合物的转位酶介导蛋白质转运到内部亚区室。

GTP 结合的 TOC 受体识别并结合传输信号。GTP 水解允许前体进入并通过 TOC 通道并进入膜间空间。

接下来，TIC 复合物允许前体转位到基质中。

基质 Hsp70 利用 ATP 水解产生的能量将新出现的肽从 TIC 复合物中拉出。

一旦完整的肽被释放到基质中，基质加工肽酶就会裂解转运信号，释放活性蛋白。

• Stroma - The fluid-filled area of a chloroplast where photosynthesis occurs.
• TOC complex - Protein complex facilitating transport across the outer chloroplast membrane.
• TIC complex - Aids protein trafficking to the inner chloroplast membrane.
• Transit peptide - Sequence that aids chloroplast-targeted protein recognition and transport.
• Thylakoid membrane - The site for light-dependent reactions in chloroplast.

• Define Stroma – It is the fluid-filled area of a chloroplast where photosynthesis happens.
• Contrast TOC vs TIC complex – Discuss key differences between the two protein complexes that aid in protein transportation in chloroplasts.
• Explore Transit peptide – Describe its role in directing proteins transport within the chloroplast.
• Explain process of Protein Transport – Discuss how proteins are transported across various chloroplast membranes.
• Apply in Chloroplast Context – Understand the functions of different chloroplast substructures and their roles in protein transport and photosynthesis.

• What is Stroma and how does it function in a chloroplast?
• What is the role of TOC and TIC complexes in chloroplast's protein transport?
• How does a Transit peptide facilitate protein transport within a chloroplast?

• Botany Students – Understand how stroma and protein transport mechanisms contribute to photosynthesis.
• Biology Teachers – Provide a clear framework for teaching protein transport in chloroplast.
• Plant Biology Researchers – Gain insight into the underlying mechanisms of chloroplast function.
• Biology Enthusiasts – Offer a deeper understanding of the fascinating micro world of plant cells.