Dynamic ring rearrangements underlie transport by MCE proteins

Mammalian cell entry (MCE) proteins mediate hydrophobic substrate transport between bacterial membranes, yet the dynamics underlying substrate translocation remain poorly understood. Here, we present the structural characterization of B. bacteriovorus Bd1152, revealing a rare two-MCE-ring architecture. Integrating X-ray crystallography, cryo-electron microscopy and continuous flexibility analysis, we uncover pronounced shearing and twisting motions between its MCE rings. Analysis of E. coli PqiB reveals that this conformational plasticity extends across distinct MCE architectures, with coordinated inter-ring rearrangements coupled to constriction and dilation of the central transport channel. These dynamics support a peristaltic model for substrate translocation through MCE proteins. Furthermore, a conserved loss-of-function PqiB mutant adopts a collapsed channel conformation, disrupting formation of a continuous transport conduit. Together, our findings establish inter-ring conformational plasticity as a conserved feature of MCE proteins and reveal how coordinated MCE-domain and pore rearrangements may facilitate lipid transport across the bacterial periplasm.