Function and molecular properties of nuclear pore-targeting complex

蛋白质的核输入是由细胞质中核定位信号（NLS）依赖复合物（称为核孔靶向复合物（PTAC））的形成引发的。PTAC是一种稳定的蛋白质复合物，由一种亲核蛋白和两种称为PTAC58/ importtin α和PTAC97/ importtin β的细胞质成分组成。输入蛋白α直接与亲核细胞的NLS结合，而输入蛋白β则为PTAC提供核孔复合物（NPC）的结合位点。我们分析了输入蛋白α和输入蛋白β的性质，得到了以下结果：我们鉴定了另外三个与已知输入蛋白α同源的小鼠基因，并表明小鼠输入蛋白α可分为三个亚家族（Rch1， NPI1和Qip1），彼此氨基酸同源性约为50%。2. 我们发现，输入蛋白α家族蛋白在识别底物方面表现出一定的特异性。例如，Qip1识别非常有限的NLS，需要上游序列和基本核心序列。更多的Q1 atp酶来识别底物。另一个证据是酪氨酸磷酸化Stat被NPI1识别，但不被Rch1识别。Stat 1显然不具有典型的碱性NLS，它与SV40 t抗原碱性NLS结合位点的NPI1部分结合不同。3. 我们研究了输入蛋白β的行为和功能域，输入蛋白β通过NPCs在细胞质和细胞核之间快速穿梭。缺失突变体的使用表明，输入蛋白β的npc易位既不需要Ran-结合，也不需要输入蛋白α-结合，而只需要该分子的npc结合结构域。输入蛋白β的NPC易位既不需要Ran也不需要其GTP水解。此外，importin β具有自身转运NPC的能力。这可能是运输因子的一个共同特征，即通过npc将某些货物运进或运出核心。4. 我们检查了β-catenin的核输入，发现b-catenin可以自行转运npc，而不需要可溶性因子如Ran。Β-catenin可能与NPC易位的输入蛋白β具有相同的特性。少

Nuclear import of proteins arc initiated by nuclear localization signal (NLS)-dependent complex formation, termed the nuclear pore-targeting complex (PTAC), in the cytoplasm. PTAC is a stable protein complex composed of a karyophilic protein and two cytosolic components termed PTAC58/importin α and PTAC97/importin β. Importin α directly binds to NLS of karyophile and importin β provides nuclear pore complex (NPC) binding site for PTAC. We analyzed the properties of importin α and importin β, and obtained the following results : 1. We have identified three additional mouse genes homologous to the known importin α, and showed that the mouse importin α can be classified into three subfamilies (Rch1, NPI1, and Qip1), showing approximately 50% amino acid identity to one another. 2. We showed that importin α family proteins show some specificities in the recognition of substrates. For example, Qip1 recognizes a very restricted NLS, requiring both the upstream sequence and basic core sequence … More of Q1 ATPase for its substrate recognition. The other evidence is that tyrosine phosphorylated Stat his recognized by NPI1, but not Rch1. Stat 1 apparently does not possess typical basic-type NLS, and it binds to different portion of NPI1 from those of SV40 T-antigen basic NLS binding site. 3. We examined the behavior and functional domain of importin β Importin β rapidly shuttles between the cytoplasm and the nucleus through NPCs. The use of deletion mutants reveals that NPC-translocation of importin β requires neither Ran- nor importin α-binding but only the NPC-binding domain of this molecule. NPC translocation of importin β was found to require neither Ran nor its GTP hydrolysis. Furthermore, importin β possesses ability to translocate NPC on its own. This could be a common feature of transport factors that carries certain cargos through NPCs into and out of the nucleus. 4. We examined the nuclear import of β-catenin and found that b-catenin can translocate NPCs on it own without requiring soluble factors including Ran. Β-catenin is likely to share the same property with importin β for NPC translocation. Less