NMR structure note: solution structure of the core domain of MESD that is essential for proper folding of LRP5/6
NMR structure note: solution structure of the core domain of MESD that is essential for proper folding of LRP5/6
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DOI：
10.1007/s10858-010-9426-8
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发表时间：
2010-08-01
影响因子：
2.7
通讯作者：
Wang, Jianjun
中科院分区：
文献类型：
--
作者：
Chen, Jianglei;Li, Qianqian;Wang, Jianjun
低密度脂蛋白受体（LDLR）家族控制多种发育和生理途径，包括内吞货物功能和信号传导能力（Herz and Bock 2002）。LDLR家族成员属于I型跨膜蛋白，其含有重复模块，包括构成配体结合结构域的富含半胱氨酸的重复序列（LDL-A）、由六个YWTD重复序列（b-螺旋桨）形成的BP结构域和表皮生长因子（EGF）重复序列。这些单元的数量和安排因家庭成员而异。LRP 5/6是Wnt信号通路中与Wnt结合的共受体，以BP结构域开始，其重复四次，随后是三个LDL-A重复。细胞外侧通过单个跨膜区段锚定在质膜上，随后是含有用于内吞作用的NPXY信号和各种细胞质衔接子和支架蛋白的相互作用基序的细胞质尾区（Strickland et al. 2002）。ER驻留的特化分子伴侣，在小鼠中称为中胚层发育（MESD）（Hsieh et al.2003），在果蝇中称为博卡（Culi and Mann 2003），对于LRP 5/6的折叠和细胞内运输至关重要。博卡是LpR 2的BP结构域成熟所特别需要的，LpR 2是LDLR的果蝇同源物（Culi et al. 2004）。野生型博卡cDNA的表达可挽救博卡的致死性W32 R突变（boca 1等位基因），表明该突变是功能丧失突变（Culi and Mann 2003）。有人建议，博卡直接结合到新合成的，新生的b-螺旋桨结构域，保持它在一个相互作用的能力状态，结合新合成的，相邻的EGF重复。在EGF结合后，BP结构域实现适当的折叠，随后消除Boca结合（Culi等人，2004）。我们发现，博卡的脊椎动物同源物MESD（12-155）不能与成熟的膜相关LRP 6结合，而C末端区域MESD（150-195）（博卡中不存在）足以与正确折叠的成熟LRP 6结合（Li et al. 2005）。这些结果表明，MESD（12-155）仅与LRP 5/6的未折叠BP结构域结合以促进其折叠。一旦BP结构域正确折叠，MESD（12-155）从成熟BP结构域解离（Culi等，2004）。因此，MESD（12-155）可以作为MESD的伴侣结构域，用于LRP 5/6的BP结构域的正确折叠。MESD截短突变体MESD（60-155）的NMR结构显示了四链反平行b-折叠的结构化核心区域和位于折叠一侧的三个a-螺旋（科勒等人，2006）。该结构缺少前59个残基，因此不能用于提供致死性boca 1等位基因的结构基础，
Low-density lipoprotein receptor (LDLR) family controls diverse developmental and physiological pathways, including an endocytic cargo function and signaling capacities (Herz and Bock 2002). LDLR family members belong to type I transmembrane proteins that contain repeating modules, including the cysteine-rich repeats (LDL-A), making up the ligand-binding domains, a BP domain formed by six YWTD repeats (b-propeller) and an epidermal growth factor (EGF) repeat. The number and arrangement of these modules vary among family members. LRP5/6, a co-receptor for binding to Wnts in the Wnt signaling pathway, start with a BP domain, which repeats four times, followed by three LDL-A repeats. The extracellular side is anchored on the plasma membrane by a single transmembrane segment, followed by a cytoplasmic tail containing NPXY signals for endocytosis and interaction motifs for a variety of cytoplasmic adaptor and scaffolding proteins (Strickland et al. 2002). An ER-resident specialized chaperone, termed mesoderm development (MESD) in mouse (Hsieh et al. 2003) and boca in Drosophila (Culi and Mann 2003), is essential for the folding and intracellular trafficking of LRP5/6. Boca is specifically required for maturation of the BP domains of LpR2, which is the Drosophila homologue of LDLR (Culi et al. 2004). A lethal W32R mutation of boca (boca1 allele) can be rescued by the expression of a wildtype boca cDNA, suggesting that this mutation is a loss-offunction mutation (Culi and Mann 2003). It is suggested Boca directly binds to the newly synthesized, nascent b-propeller domain, maintaining it in an interaction competent state for binding of the newly synthesized, adjacent EGF-repeat. Upon EGF-binding, the BP domain achieves a proper fold, subsequently eliminating Boca-binding (Culi et al. 2004). We showed that a vertebrates homologue of Boca, MESD (12–155), fails to bind to mature, membraneassociated LRP6, whereas the C-terminal region, MESD (150–195), which is absent in Boca, is sufficient for binding to the properly folded, mature LRP6 (Li et al. 2005). These results suggest that MESD (12–155) only binds to unfolded BP-domains of LRP5/6 to promote their folding. Once the BP domains fold properly, MESD (12–155) dissociates from the mature BP domain (Culi et al. 2004). Thus, MESD (12–155) may serve as a chaperone domain of MESD for proper folding of the BP domains of LRP5/6. The NMR structure of a MESD truncation mutant, MESD (60–155) showed a structured core region of a fourstranded anti-parallel b-sheet and three a-helices positioned in one side of the sheet (Kohler et al. 2006). This structure lacks the first 59-residues, thus can’t be used to provide structural basis of the lethal boca1 allele which