Conti E, Kuriyan J
Laboratory of Molecular Biophysics, Howard Hughes Medical Institute, The Rockefeller University, New York 10021, USA.
Structure. 2000 Mar 15;8(3):329-38. doi: 10.1016/s0969-2126(00)00107-6.
Karyopherin alpha (importin alpha) is an adaptor molecule that recognizes proteins containing nuclear localization signals (NLSs). The prototypical NLS that is able to bind to karyopherin alpha is that of the SV40 T antigen, and consists of a short positively charged sequence motif. Distinct classes of NLSs (monopartite and bipartite) have been identified that are only partly conserved with respect to one another but are nevertheless recognized by the same receptor.
We report the crystal structures of two peptide complexes of yeast karyopherin alpha (Kapalpha): one with a human c-myc NLS peptide, determined at 2.1 A resolution, and one with a Xenopus nucleoplasmin NLS peptide, determined at 2.4 A resolution. Analysis of these structures reveals the determinants of specificity for the binding of a relatively hydrophobic monopartite NLS and of a bipartite NLS peptide. The peptides bind Kapalpha in its extended surface groove, which presents a modular array of tandem binding pockets for amino acid residues.
Monopartite and bipartite NLSs bind to a different number of amino acid binding pockets and make different interactions within them. The relatively hydrophobic monopartite c-myc NLS binds extensively at a few binding pockets in a similar manner to that of the SV40 T antigen NLS. In contrast, the bipartite nucleoplasmin NLS engages the whole array of pockets with individually more limited but overall more abundant interactions, which include the NLS two basic clusters and the backbone of its non-conserved linker region. Versatility in the specific recognition of NLSs relies on the modular.
核转运蛋白α（输入蛋白α）是一种衔接分子，可识别含有核定位信号（NLSs）的蛋白质。能够与核转运蛋白α结合的典型NLS是SV40 T抗原的NLS，由一段短的带正电荷序列基序组成。已鉴定出不同类型的NLS（单分型和双分型），它们彼此之间仅部分保守，但仍被同一受体识别。
我们报道了酵母核转运蛋白α（Kapα）的两种肽复合物的晶体结构：一种与人类c-myc NLS肽形成的复合物，分辨率为2.1 Å；另一种与非洲爪蟾核质蛋白NLS肽形成的复合物，分辨率为2.4 Å。对这些结构的分析揭示了相对疏水的单分型NLS和双分型NLS肽结合特异性的决定因素。这些肽在Kapα的延伸表面凹槽中结合，该凹槽呈现出一系列串联的氨基酸结合口袋。
单分型和双分型NLS与不同数量的氨基酸结合口袋结合，并在其中形成不同的相互作用。相对疏水的单分型c-myc NLS以与SV40 T抗原NLS相似的方式在少数几个结合口袋中广泛结合。相比之下，双分型核质蛋白NLS与整个口袋阵列结合，单个相互作用更有限，但总体上更丰富，包括NLS的两个碱性簇及其非保守连接区的主链。NLS特异性识别的通用性依赖于模块化。
