Department of Plant Biology & Pathology, Rutgers University, New Brunswick, NJ, USA.
Plant Signal Behav. 2011 Dec;6(12):2051-6. doi: 10.4161/psb.6.12.18247.
Plant metacaspases (MCPs) are conserved cysteine proteases that have been postulated as regulators of programmed cell death (PCD). Although MCPs have been proven to have PCD relevant functions in multiple species ranging from fungi to plants, how these proteases are modulated in vivo remains unclear. Aside from demonstrating that these proteases are distinct from metazoan caspases due to their different target site specificities, how these proteases are used to tightly regulate cell death progression is a key question that remains to be resolved. Some recent studies on the biochemical characteristics of type-II MCP activities in Arabidopsis may begin to shed additional light on this aspect. The in vitro catalytic activities of recombinant AtMC4, AtMC5 and AtMC8 are found to be Ca(2+)-dependent while recombinant AtMC9 is active under mildly acidic conditions and not dependent on stimulation by Ca(2+). Alterations of cellular pH and Ca(2+) concentration commonly occur during various stresses and may help to orchestrate differential activation of latent MCPs under these conditions. Recent peptide mapping for recombinant AtMC4 (also called Metacaspase-2d) followed by site-specific mutagenesis studies have revealed multiple potential self-cleavage sites with the identification of a conserved lysine residue (Lys-225) as the key position for enzyme function both in vitro and in vivo. The multiple self-cleavage sites in MCPs may also facilitate desensitization of these proteases and can provide a means for fine-tuning their proteolytic activities in order to achieve more sensitive control of downstream processes.
植物的 metacaspases (MCPs) 是保守的半胱氨酸蛋白酶，被认为是程序性细胞死亡 (PCD) 的调节剂。尽管 MCPs 已被证明在从真菌到植物的多种物种中具有与 PCD 相关的功能，但这些蛋白酶在体内是如何被调节的仍然不清楚。除了证明这些蛋白酶由于其不同的靶位特异性与后生动物的半胱天冬酶不同之外，这些蛋白酶如何被用来严格调节细胞死亡进程是一个关键问题，有待解决。最近对拟南芥 II 型 MCP 活性的生化特性的一些研究可能开始进一步阐明这一方面。发现重组 AtMC4、AtMC5 和 AtMC8 的体外催化活性依赖于 Ca(2+)，而重组 AtMC9 在酸性条件下具有活性，不依赖于 Ca(2+)的刺激。在各种应激过程中，细胞内 pH 和 Ca(2+)浓度的改变通常会发生，这可能有助于协调潜伏 MCPs 在这些条件下的差异激活。最近对重组 AtMC4（也称为 Metacaspase-2d）进行的肽图谱分析以及随后的定点突变研究表明，多个潜在的自我切割位点，鉴定出一个保守的赖氨酸残基（Lys-225）作为酶功能的关键位置，无论是在体外还是体内。MCP 中的多个自我切割位点也可能使这些蛋白酶脱敏，并为精细调节它们的蛋白水解活性提供一种手段，以实现对下游过程的更敏感控制。
