HECTD1 is both a positive regulator and substrate of caspase-3 activity during apoptotic cell death

Programmed cell death is a complex and tightly regulated sequence of events that determines cell fate during tissue homeostasis, development, and pathogenesis. The small protein modifier ubiquitin mediates important regulatory functions during cell death by regulating the stability and activity of checkpoint proteins and the assembly of cell death signalling complexes. The caspase family of cysteine aspartases are essential effectors of apoptotic cell death. Components of the ubiquitin system including RING ubiquitin ligases XIAP, MDM2, RBX1; RBR E3 ubiquitin ligases Parkin and LUBAC; and HECT E3 ubiquitin ligases NEDD4 and Itch are also substrates of caspase-mediated cleavage. In the case of NEDD4 and Itch, the single cleavage event occurs outside of the catalytic HECT domain and it remains unclear whether such cleavage events impact on ubiquitin ligase activity and/or function. Here, we identified the E3 ubiquitin ligase HECTD1 as the third HECT E3 cleaved by caspase-mediated cleavage during apoptotic cell death, in a manner which does not affect the integrity of the catalytic C-ter HECT domain. We mapped the single cleavage event to DFLD1664{downarrow}S and showed that the cleaved C-ter product, which contains the HECT ligase domain, is as stable as the endogenous full length protein. We also found that HECTD1 transient depletion led to reduced caspase-3 activity, but not caspase 8 nor 9. Furthermore, we also identified caspase-3 as the protease responsible for HECTD1 cleavage at Asp1664 suggesting that HECTD1 and caspase-3 might be part of a novel feedback loop mechanism during apoptotic cell death. This study highlight novel crosstalk between cell death mechanisms and the ubiquitin system and raises important questions on whether proteolytic cleavage of E3 ubiquitin ligases might represent an underappreciated mode of regulation during cell death mechanisms.