The GspLM inner membrane complex from the bacterial type II secretion system is a dimer of dimers and interacts with the system ATPase with high affinity

ABSTACTThe type II secretion system (T2SS) is a multiprotein machinery spanning the diderm of gram-negative bacteria. T2SS contributes to the virulence of numerous gram-negative pathogens, including the multidrug resistant species Pseudomonas aeruginosa, Acinetobacter baumanii, Klebsiella pneumonia and Vibrio cholerae. Even though the T2SS has been studied extensively over the past three decades, our understanding of the molecular basis of its biogenesis and of its overall structure still remains unclear. Here we show that the core component of the inner membrane platform, the GspLM membrane protein complex, can be isolated as a dimer of dimers. Importantly, the complex is able to bind the T2SS ATPase, GspE, with high affinity. Finally, we have developed single domain VHH camelid antibodies (nanobodies) against the GspLM complex and have identified a nanobody that effectively prevents the cytoplasmic domain of GspL, GspLcyto, from binding to GspE. Our findings suggest that the T2SS ATPase is permanently associated with the inner membrane platform and that the GspELM complex should be considered as a key subassembly for the biogenesis of the T2SS apparatus.