Spatially restricting caspase activation

In addition to executing programmed cell death, members of the caspase family of cysteine proteases also mediate the degradation of organelles and other cellular structures (see Minis and Steller). As the interconnected spermatids of Drosophilamelanogaster differentiate into mature sperm, they undergo a caspase-dependent process called “individualization,” during which the spermatids are separated and most of their cytoplasm and organelles are degraded. Aram etal. found that the β subunit of the ATP-specific form of succinyl-CoA synthetase (A-Sβ), a component of the Krebs cycle, restricts caspase activity to areas immediately around mitochondria. In yeast two-hybrid assays and coimmunoprecipitation experiments with extracts from cultured Drosophila S2 cells, A-Sβ interacted with Cul3T and Klhl10, two components of the testis-specific form of the Cullin-RING ubiquitin ligase (CRL3) complex that activates caspases. Expression of a testis-specific, long isoform of A-Sβ (A-SβT) increased during spermatid individualization and was required for caspase activation during individualization. A-SβT activated the CRL3 complex when the complex was reconstituted in S2 cells or ectopically expressed in the eye. Whereas the somatic, short form of A-Sβ (A-SβS) localizes to the mitochondrial matrix, A-SβT was present on the mitochondrial outer membrane and recruited Cul3 and Klhl10 to the mitochondrial surface. The authors mapped the domains of A-SβT that mediated binding to CRL3 and showed that A-SβT competed with an endogenous CRL3 inhibitor for binding to CRL3. Thus, A-SβT limits CRL3 activation to the mitochondrial surface, which in turn restricts caspase activation to this location to ensure that mitochondria are eliminated without triggering death of the spermatid.L. Aram, T. Braun, C. Braverman, Y. Kaplan, L. Ravid, S. Levin-Zaidman, E. Arama, A Krebs cycle component limits caspase activation rate through mitochondrial surface restriction of CRL activation. Dev.Cell37, 15–33 (2016). [PubMed]A. Minis, H. Steller, Krebs cycle moonlights in caspase regulation. Dev.Cell37, 1–2 (2016). [PubMed]