Structural Plasticity of the Membrane-Bound Protein Degradation Assembly Supports Bacterial Adaptation to Stress

Protein degradation by AAA+ proteases is essential for bacterial adaptation to environmental stress. The membrane-bound AAA+ protease FtsH forms a large inner-membrane complex with the SPFH (Stomatin, Prohibitin, Flotillin, HflK/C) family transmembrane proteins HflK and HflC, playing a key role in bacterial recovery from aminoglycoside antibiotic stress. Recent structural studies have revealed both open, asymmetric and closed, symmetric conformations of the HflK/C assembly under different sample-preparation conditions, suggesting two distinct models for how this complex modulates FtsH proteolysis. To determine which conformation reflects the biologically active state, we engineered a disulfide-crosslinked HflK/C variant to stabilize the closed conformation and resolved its structure using high-resolution cryo-EM. Phenotypic assays showed that cells expressing either this stabilized, closed HflK/C variant or an HflK/C mutant that disrupts interactions with FtsH exhibit significantly impaired growth under aminoglycoside stress. Surprisingly, the cryo-EM structure of the FtsH*HflK/C complex from cells challenged with the aminoglycoside antibiotic tobramycin revealed a novel HflK/C arrangement, characterized by two openings on opposite sides that may facilitate substrate access to FtsH during proteotoxic stress. Together, our results suggest that both the dynamic open conformation of HflK/C and its specific interactions with FtsH are critical for adaptation to aminoglycoside-induced stress. Given the conserved structural and functional features of SPFH family members, our findings may offer a broader framework for understanding how this protein family operates under both basal and stress conditions.