Selective degradation of excess Ldb1 by Rnf12/RLIM confers proper Ldb1 expression levels and Xlim-1/Ldb1 stoichiometry in Xenopus organizer functions
Selective degradation of excess Ldb1 by Rnf12/RLIM confers proper Ldb1 expression levels and Xlim-1/Ldb1 stoichiometry in Xenopus organizer functions
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DOI：
10.1242/dev.00621
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发表时间：
2003-09-01
期刊：
影响因子：
4.6
通讯作者：
Taira, M
中科院分区：
文献类型：
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作者：
Hiratani, I;Yamamoto, N;Taira, M
非洲爪蟾LIM同源结构域（LIM-HD）蛋白，Xlim-1，在Spemann组织者中表达，并与其正调控因子Ldb 1合作，以激活组织者基因表达。虽然这种激活可能是通过Xlim-1/Ldb 1四聚体形成介导的，但调节适当Xlim-1/Ldb 1化学计量的机制在很大程度上仍然未知。我们分离了RING指蛋白Rnf 12/RLIM的爪蟾直系同源物（XRnf 12），并探讨了其与Xlim-1和Ldb 1的功能相互作用。虽然XRnf 12作为Ldb 1的E3泛素连接酶发挥作用并导致Ldb 1的蛋白酶体依赖性降解，但我们发现高水平Xlim-1的共表达抑制了XRnf 12对Ldb 1的降解。这种抑制需要Xlim-1的LIM结构域和Ldb 1的LIM相互作用结构域，这表明Ldb 1在与Xlim-1结合时逃脱了XRnf 12的降解。我们进一步表明，高水平的Ldb 1抑制Xlim-1/Ldb 1的组织者活性，这表明过量的Ldb 1分子干扰Xlim-1/Ldb 1的化学计量。与此相一致，Ldb 1在背侧边缘区的过表达抑制了几个组织者基因的表达，包括假定的Xlim-1靶点，重要的是，这种抑制被XRnf 12的共表达所拯救。这些数据表明，XRnf 12赋予适当的Ldb 1蛋白水平和Xlim-1/Ldb 1化学计量为他们的功能，在组织者。再加上在整个早期胚胎发生Ldb 1和XRnf 12的表达模式的相似性，我们建议Rnf 12/RLIM作为一个特定的调节Ldb 1，以确保其适当的相互作用与LIM-HD蛋白和可能的其他Ldb 1相互作用的蛋白质在组织者以及在其他组织。
The Xenopus LIM homeodomain (LIM-HD) protein, Xlim-1, is expressed in the Spemann organizer and cooperates with its positive regulator, Ldb1, to activate organizer gene expression. While this activation is presumably mediated through Xlim-1/Ldb1 tetramer formation, the mechanisms regulating proper Xlim-1/Ldb1 stoichiometry remains largely unknown. We isolated the Xenopus ortholog (XRnf12) of the RING finger protein Rnf12/RLIM and explored its functional interactions with Xlim-1 and Ldb1. Although XRnf12 functions as a E3 ubiquitin ligase for Ldb1 and causes proteasome-dependent degradation of Ldb1, we found that co-expression of a high level of Xlim-1 suppresses Ldb1 degradation by XRnf12. This suppression requires both the LIM domains of Xlim-1 and the LIM interaction domain of Ldb1, suggesting that Ldb1, when bound to Xlim-1, escapes degradation by XRnf12. We further show that a high level of Ldb1 suppresses the organizer activity of Xlim-1/Ldb1, suggesting that excess Ldb1 molecules disturb Xlim-1/Ldb1 stoichiometry. Consistent with this, Ldb1 overexpression in the dorsal marginal zone suppresses expression of several organizer genes including postulated Xlim-1 targets, and importantly, this suppression is rescued by co-expression of XRnf12. These data suggest that XRnf12 confers proper Ldb1 protein levels and Xlim-1/Ldb1 stoichiometry for their functions in the organizer. Together with the similarity in the expression pattern of Ldb1 and XRnf12 throughout early embryogenesis, we propose Rnf12/RLIM as a specific regulator of Ldb1 to ensure its proper interactions with LIM-HD proteins and possibly other Ldb1-interacting proteins in the organizer as well as in other tissues.