Melen Gustavo J, Levy Sagi, Barkai Naama, Shilo Ben-Zion
Department of Molecular Genetics, Weizmann Institute of Science, Rehovot, Israel.
Mol Syst Biol. 2005;1:2005.0028. doi: 10.1038/msb4100036. Epub 2005 Dec 13.
Translating a graded morphogen distribution into tight response borders is central to all developmental processes. Yet, the molecular mechanisms generating such behavior are poorly understood. During patterning of the Drosophila embryonic ventral ectoderm, a graded mitogen-activated protein kinase (MAPK) activation is converted into an all-or-none degradation switch of the Yan transcriptional repressor. Replacing the cardinal phosphorylated amino acid of Yan by a phosphomimetic residue allowed its degradation in a MAPK-independent manner, consistent with Yan phosphorylation being the critical event in generating the switch. Several alternative threshold mechanisms that could, in principle, be realized by this phosphorylation, including first order, cooperativity, positive feedback and zero-order ultrasensitivity, were analyzed. We found that they can be distinguished by their kinetics and steady-state responses to Yan overexpression. In agreement with the predictions for zero-order kinetics, an increase in Yan levels did not shift the degradation border, but significantly elevated the time required to reach steady state. We propose that a reversible loop of Yan phosphorylation implements a zero-order ultrasensitivity-like threshold mechanism, with the capacity to form sharp thresholds that are independent of the level of Yan.
将分级形态发生素分布转化为紧密的反应边界是所有发育过程的核心。然而，产生这种行为的分子机制却知之甚少。在果蝇胚胎腹侧外胚层的模式形成过程中，分级的丝裂原活化蛋白激酶（MAPK）激活被转化为Yan转录抑制因子的全或无降解开关。用模拟磷酸化的残基取代Yan的主要磷酸化氨基酸，使其能够以不依赖MAPK的方式降解，这与Yan磷酸化是产生开关的关键事件一致。分析了几种原则上可以通过这种磷酸化实现的替代阈值机制，包括一级、协同性、正反馈和零级超敏感性。我们发现，它们可以通过其动力学和对Yan过表达的稳态反应来区分。与零级动力学的预测一致，Yan水平的增加并没有改变降解边界，但显著延长了达到稳态所需的时间。我们提出，Yan磷酸化的可逆环实现了一种类似零级超敏感性的阈值机制，能够形成与Yan水平无关的尖锐阈值。
