Bacteria employ large protein complexes called specialized 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 specialized 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?
One of the proteins in the A-Q complex is SpoIIIAG, a protein with remote homology to the EscJ/PrgK family of ring-forming proteins found in Type III secretion systems. We show using Bacillus subtilis that the extracellular domain of SpoIIIAG assembles into a 24-member ring, with a large (6 nm) pore, of similar architecture and dimensions as those found in Type III secretion systems. Furthermore, mutations that abrogate ring formation in vitro impair spore development in vivo.
Collectively, this work provides the first direct evidence that the A-Q complex assembles a conduit between the mother cell and spore. Furthermore, it highlights a new role for specialized secretion systems in spore development.