The mechanism of cell-cycle-dependent proteasomal degradation of archaeal ESCRT-III homolog CdvB in Sulfolobus

Protein degradation orders events in the cell division cycle in eukaryotes, bacteria, and archaea. In eukaryotes, chromosome segregation and mitotic exit are triggered by proteasome-dependent degradation of securin and cyclin B, respectively. Recent findings show that the archaeal proteasome also targets substrates, including CdvB, for degradation in a cell-cycle-dependent manner in Sulfolobus acidocaldarius-an experimentally tractable archaeal relative of eukaryotes. Here, using CdvB as a model substrate to explore the mechanism of cyclic protein degradation, we demonstrate that the C-terminal broken-winged helix of CdvB, previously shown to bind CdvA, is sufficient to render a fusion protein unstable as cells progress through division. We show that the rate of CdvB degradation accelerates during division in part due to a cell-cycle-dependent increase in expression of the proteasome-activating nucleotidase (PAN), under the control of a cyclically expressed novel transcription factor "CCTF1" that represses PAN expression. Taken together, these findings reveal mechanisms by which archaea, despite lacking cyclin-dependent kinases, control proteasome-mediated degradation to order events during cell division.