The chemistry and physics of cellular shutdown: unraveling how and why cells enter into a hypometabolic state

生命体的一个标志是它的高度动态和精致的组织状态。维持这种微妙的状态需要持续的能量输入和远离热力学平衡的新陈代谢。然而，生物体通常生活在不可预测的环境中，并经常经历不利于生长和繁殖的条件。在这种情况下，细胞可以通过进入代谢活动减少的非分裂状态(低代谢状态)来保护自己。然而，细胞如何进入这种状态并从中恢复在很大程度上是一个悬而未决的问题。最近在萌芽酵母中的发现表明，饥饿和其他应激条件导致细胞质广泛重排，并将关键代谢酶组装成更高级别的结构。在这项拨款提案中要检验的假设是，这些变化是由细胞液pH的下降引起的，并且由pH引起的细胞质组织的变化促进进入保护性的低代谢状态。为了证明这一假设，我们将调查饥饿诱导的酶组装是如何形成的(它们的组装机制)，以及组装形成如何影响这些酶的活性(它们的分子和细胞功能)。我们的初步发现表明，翻译起始因子eIF2B组装到丝状结构中在关闭蛋白质合成方面发挥了关键作用。因此，我们将特别关注eIF2B组装如何促进进入低代谢状态的问题。我们将进一步研究细胞质是否会因应激而改变其物理性质。我们已经有强有力的证据表明，当能量耗尽时，细胞质从动态转变为冻结状态，我们的目标是确定细胞质冻结的分子和结构原因。最后，我们的目标是证明这些分子变化提高了酵母细胞在能量耗尽和其他类型的压力下的生存和寿命。我们的研究将对理解不同的生理状态，如细胞休眠和静止具有广泛的意义。他们还将揭示真核细胞如何应对严重的环境扰动。我们还预测，我们的研究提供了关于代谢性疾病和衰老的潜在原因和后果的重要线索，并将揭示生物体在死亡时经历的关键分子变化。

A hallmark of living matter is its highly dynamic and yet exquisitely organized state. Maintenance of this delicate state requires a constant input of energy and a metabolism that is far from thermodynamic equilibrium. However, organisms typically live in unpredictable environments and frequently experience conditions that are not optimal for growth and reproduction. Under such conditions, cells can protect themselves by entering into a non-dividing state with reduced metabolic activity (hypometabolic state). However, how cells enter into and recover from this state is a largely unresolved question.Recent findings in budding yeast suggest that starvation and other stress conditions induce an extensive rearrangement of the cytoplasm and the assembly of key metabolic enzymes into higher order structures. The hypothesis to be tested in this grant proposal is that these changes are induced by a drop in cytosolic pH and that the pH-induced alterations in cytoplasmic organization promote entry into a protective hypometabolic state. To prove this hypothesis, we will investigate how starvation-induced enzyme assemblies form (their mechanism of assembly) and how assembly formation affects the activity of these enzymes (their molecular and cellular function). Our preliminary findings suggest that assembly of the translation initiation factor eIF2B into filamentous structures plays a key role in shutting down protein synthesis. Hence, we will particularly focus on the question of how eIF2B assembly promotes entry into a hypometabolic state. We will further investigate whether the cytoplasm changes its physical properties in response to stress. We already have strong evidence that the cytoplasm transitions from a dynamic to a frozen state upon energy depletion, and we aim to identify the molecular and structural causes of cytoplasmic freezing. Finally, we aim to demonstrate that these molecular changes improve the survival and longevity of yeast cells in response to energy depletion and other types of stresses.Our studies will have broad implications for understanding alternative physiological states, such as cellular dormancy and quiescence. They will also reveal how a eukaryotic cell can deal with severe environmental perturbations. We also predict that our studies provide important clues about potential causes and consequences of metabolic diseases and aging, and will reveal critical molecular changes that an organism undergoes when it dies.