Bacteria employ large protein complexes called specialised secretion systems to transport proteins across their envelope. These nanomachines are notorious in Gram-negative pathogens where they play important roles in the infection of host cells. In this instance, they transverse the inner and outer membranes and utilize ATP hydrolysis in the cytoplasm to energize secretion of proteins across the outer membrane. Interestingly, the problem of transporting proteins across two membranes is not unique to Gram-negative bacteria. In Gram-positive, spore-forming bacteria such as the model Bacillus subtilis and the pathogen Clostridium difficile, a double-membrane assembles around the developing spore: one membrane derived from the mother cell and another from the spore. A protein complex (called the A-Q complex) with remote homology to specialised secretion systems spans these two membranes and is essential for spore development. This transenvelope complex has been hypothesized to function as a channel for molecular transport between these two cells.

There are many outstanding questions surrounding the A-Q complex: Is it a secretion complex and what does it secrete? What does it look like? How is it assembled? Do we even have the complete parts list for this complex? 

In this talk, I will share seminal work that has helped define the A-Q complex as a new type of specialised secretion system with a specific role during bacterial sporulation.