Department of Pharmaceutical Chemistry and Cellular &Molecular Pharmacology, University of California, San Francisco, CA, USA.
Cell Death Differ. 2017 Aug;24(8):1380-1389. doi: 10.1038/cdd.2017.44. Epub 2017 May 12.
Protease biology is intimately linked to the functional consequences of substrate cleavage events. Human caspases are a family of 12 fate-determining cysteine proteases that are best known for driving cell death, either apoptosis or pyroptosis. More recently, caspases have been shown to be involved in other cellular remodeling events as well including stem cell fate determination, spermatogenesis, and erythroid differentiation. Recent global proteomics methods enable characterization of the substrates that caspases cleave in live cells and cell extracts. The number of substrate targets identified for individual caspases can vary widely ranging from only a (few) dozen targets for caspases-4, -5, -9, and -14 to hundreds of targets for caspases-1, -2, -3, -6, -7, and -8. Proteomic studies characterizing the rates of target cleavage show that each caspase has a preferred substrate cohort that sometimes overlaps between caspases, but whose rates of cleavage vary over 500-fold within each group. Determining the functional consequences of discrete proteolytic events within the global substrate pool is a major challenge for the field. From the handful of individual targets that have been studied in detail, there are only a few so far that whose single cleavage event is capable of sparking apoptosis alone, such as cleavage of caspase-3/-7 and BIM, or for pyroptosis, gasdermin D. For the most part, it appears that cleavage events function cooperatively in the cell death process to generate a proteolytic synthetic lethal outcome. In contrast to apoptosis, far less is known about caspase biology in non-apoptotic cellular processes, such as cellular remodeling, including which caspases are activated, the mechanisms of their activation and deactivation, and the key substrate targets. Here we survey the progress made in global identification of caspase substrates using proteomics and the exciting new avenues these studies have opened for understanding the molecular logic of substrate cleavage in apoptotic and non-apoptotic processes.
蛋白酶生物学与底物切割事件的功能后果密切相关。人类半胱氨酸蛋白酶家族（caspases）由 12 种命运决定的半胱氨酸蛋白酶组成，它们最著名的作用是驱动细胞死亡，包括细胞凋亡或细胞焦亡。最近，研究表明，caspases 还参与了其他细胞重塑事件，包括干细胞命运决定、精子发生和红细胞分化。最近的全局蛋白质组学方法能够描述在活细胞和细胞提取物中被 caspase 切割的底物。单个 caspase 的底物靶标数量差异很大，从 caspase-4、-5、-9 和 -14 仅有几十个靶标，到 caspase-1、-2、-3、-6、-7 和 -8 有数百个靶标不等。对靶标切割速率进行特征描述的蛋白质组学研究表明，每个 caspase 都有一个首选的底物群，这些底物群有时在 caspase 之间重叠，但在每个组内的切割速率差异超过 500 倍。确定全局底物库中离散蛋白水解事件的功能后果是该领域的主要挑战。到目前为止，在少数已被详细研究的单个靶标中，只有少数单个切割事件能够单独引发细胞凋亡，例如 caspase-3/-7 和 BIM 的切割，或细胞焦亡，如 gasdermin D。在大多数情况下，切割事件在细胞死亡过程中协同作用，产生一种蛋白水解的合成致死结果。与细胞凋亡相反，关于非细胞凋亡细胞过程（如细胞重塑）中的 caspase 生物学，我们知之甚少，例如哪些 caspase 被激活、它们的激活和失活机制，以及关键的底物靶标。在这里，我们综述了使用蛋白质组学对 caspase 底物进行全局鉴定所取得的进展，以及这些研究为理解细胞凋亡和非细胞凋亡过程中底物切割的分子逻辑开辟的令人兴奋的新途径。
