A C-terminal Processing Protease Implicated in Flagellin Turnover and Developmental Progression in a Bacterial Predator

Carboxy-terminal processing proteases (CTPs) are widely conserved bacterial proteases implicated in protein maturation, quality control, and stress responses, yet many family members functions remain unclear. Here, we characterise Bd0967, a previously unstudied CTP from the predatory bacterium Bdellovibrio bacteriovorus, and its role in flagellar function during predatory development. Crystal structures reveal a self-compartmentalised protease in which a PDZ domain forms a lid over a large internal cavity accessed through a proteolytic tunnel. Co-purifying peptides occupied two distinct substrate-binding sites, suggesting a coordinated recognition mechanism. Affinity pulldowns coupled with mass spectrometry identified several Bdellovibrio flagellins as candidate substrates, which were subsequently validated by biochemical assays and shown to be selectively degraded through recognition of a conserved C-terminal motif. A Bd0967-mCherry translational fusion localised to periplasmic foci during the intracellular predatory growth, consistent with flagellar resorption and flagellin turnover following prey invasion. Deletion of bd0967 caused developmental defects, including aberrant Bdellovibrio cell morphology and reduced predation efficiency. Together, these findings establish Bd0967 as a specialised CTP that couples flagellin degradation and developmental progression in a predatory bacterium.