1、secretory pathway n线粒体具有1000多种蛋白质，其自身合成的仅十余种，因此大量蛋白质是在细胞质中合成的。线粒体前体蛋白质在在运输以前，以未折叠的形式存在，N端有一段信号序列称为导肽或引肽，完成转运后被信号肽酶（signal peptidase）切除，就成为成熟蛋白，这种现象就叫做后转译（posttranslation）。信号肽的特点是：多位于肽链的N端，由大约20个氨基酸构成；形成一个两性螺旋，带正电荷的氨基酸残基和不带电荷的疏水氨基酸残基分别位于螺旋的两侧；对所牵引的蛋白质没有特异性要求。MatrixOuter membraneIntermembrane spaceCro
2、ss section of mitochondrionInner membrane Protein import into mitochondriaProtein Transport into Inner Membrane and Intermembrane Space Requires Two Signal Sequences线粒体转运与叶绿体非常相似线粒体转运与叶绿体非常相似叶绿体蛋白转运运输机理与线粒体的相似。如：都发生后转译；每一种膜上有特定的转位因子；具有接触点（contact site）；需要能量，同样利用ATP和质子动力势；前体蛋白N端有信号序列，使用后被信号肽酶切除。但两者的蛋
3、白质转运体系中除了某些hsp分子相同外，转位因子复合体是不同的。Import of rubisco small(S)subunits into the chloroplast stroma 转运肽内囊体膜蛋白转运需要两段信号内囊体膜蛋白转运需要两段信号PTS1PTS2The nuclear pore complex0.1um Side view of nuclear poreMechanism of protein importProteins selected for import into nucleus have a nuclear localization signal(NLS)e.g
4、.-Pro-Pro-Lys-Lys-Lys-Arg-NLS is recognized by cytosolic nuclear import receptors which bind to nuclear pore fibrils extending into cytosolPore opens and protein plus import receptor enter nucleus.Import receptor exported for re-useNuclear localization signals Nuclear localisation signal(NLS)Nuclear
5、 localization signals Mechanism of transport through nuclear poresNLS need not cut after transport 1.4.2 Mechanism of export from nucleusMost of traffic moving out of nucleus consists of different types of RNA moleculesRNA moves through nuclear pore as complex of ribonucleoprotein(RNP):核糖核蛋白颗粒核糖核蛋白颗
6、粒Protein component of RNP contains a nuclear export signal(NES)that is recognised by export proteinsmRNA is bound by hnRNP only after fully spliced so only mature RNA is exportedmRNA export 核内不均一核糖核蛋白核内不均一核糖核蛋白(heterogeneous ribonucleoprotein,hnRNP)信使信使RNP(messenger RNP,mRNP)mRNA的输出的输出nuclear proteinsnRNA export snRNAsnRNA export export Nuclear exportProtein importMaterial export
