Molecular Insights into CLD Domain Dynamics and Toxin Recruitment of the HlyA E. coli T1SS

Abstract

Escherichia coli is a Gram-negative opportunistic pathogen causing nosocomial infections through the production of various virulence factors. Type 1 secretion systems (T1SS) contribute to virulence by mediating one-step secretion of unfolded substrates directly into the extracellular space, bypassing the periplasm. A well-studied example is the hemolysin A (HlyA) system, which secretes the HlyA toxin in an unfolded state across the inner and outer membranes. T1SS typically comprise a homodimeric ABC transporter (HlyB), a membrane fusion protein (HlyD), and the outer membrane protein TolC. Some ABC transporters in T1SS also contain N-terminal C39 peptidase or peptidase-like (CLD) domains implicated in substrate interaction. Recent cryo-EM studies have resolved the inner-membrane complex as a trimer of HlyB homodimers with associated HlyD protomers. However, a full structural model including TolC remains unavailable. We present the first complete structural model of the HlyA T1SS, constructed using template- and MSA-based information and validated by SAXS. Molecular dynamics simulations provide insights into the function of the CLD domains, which are partially absent from existing cryo-EM structures. These domains may modulate transport by stabilizing specific conformations of the complex. Simulations with a C-terminal fragment of HlyA indicate that toxin binding occurs in the occluded conformation of HlyB, potentially initiating substrate transport through a single HlyB protomer before transitioning to an inward-facing state. HlyA binding also induces allosteric effects on HlyD, altering key residues involved in TolC recruitment. These results indicate how substrate recognition and transport are coupled and may support the development of antimicrobial strategies targeting the T1SS.