Department of Chemistry, University of Massachusetts, 104 LGRT, 710 N. Pleasant Street, Amherst, MA, 01003, USA.
Cell Death Differ. 2018 Jun;25(6):1025-1039. doi: 10.1038/s41418-017-0052-9. Epub 2018 Jan 19.
Caspases, the cysteine proteases which facilitate the faithful execution of apoptosis, are tightly regulated by a number of mechanisms including phosphorylation. In response to cAMP, PKA phosphorylates caspase-9 at three sites preventing caspase-9 activation, and suppressing apoptosis progression. Phosphorylation of caspase-9 by PKA at the functionally relevant site Ser-183 acts as an upstream block of the apoptotic cascade, directly inactivating caspase-9 by a two-stage mechanism. First, Ser-183 phosphorylation prevents caspase-9 self-processing and directly blocks substrate binding. In addition, Ser-183 phosphorylation breaks the fundamental interactions within the caspase-9 core, promoting disassembly of the large and small subunits. This occurs despite Ser-183 being a surface residue distal from the interface between the large and small subunits. This phosphorylation-induced disassembly promotes the formation of ordered aggregates around 20 nm in diameter. Similar aggregates of caspase-9 have not been previously reported. This two-stage regulatory mechanism for caspase-9 has likewise not been reported previously but may be conserved across the caspases.
半胱天冬氨酸蛋白酶（caspases）是一种能够确保细胞凋亡精确执行的半胱氨酸蛋白酶，其活性受到多种机制的严格调控，包括磷酸化。在环磷酸腺苷（cAMP）的刺激下，蛋白激酶 A（PKA）能够在三个特定位点对 caspase-9 进行磷酸化，从而抑制 caspase-9 的激活，阻止细胞凋亡的进展。PKA 在 caspase-9 的 Ser-183 这一功能相关位点上进行的磷酸化反应能够阻断凋亡级联反应，直接对 caspase-9 进行失活调控。该磷酸化反应通过两步机制实现：首先，Ser-183 的磷酸化会阻止 caspase-9 的自身酶切，并直接阻断底物的结合。此外，Ser-183 的磷酸化会破坏 caspase-9 核心区域内的基本相互作用，从而促进大小亚基的解离。尽管 Ser-183 是远离大小亚基界面的表面残基，但上述两种相互作用都会发生。这种磷酸化诱导的解离会促进直径约为 20nm 的有序聚集体的形成。此前从未报道过 caspase-9 的这种聚集现象。这种两步式的 caspase-9 调控机制此前也从未被报道过，但它可能在整个胱天冬酶家族中都具有保守性。
