1.A.34 The Bacillus Gap Junction-like Channel-forming Complex (GJ-CC) Family
SpoIIQ and SpoIIIAH (See 9.B.70.1.1) have been reported to form a channel connecting the B. subtilis mother cell to the pre-forespore. SpoIIIAA-AH are encoded within a single operon and may function as a protein secretion system. SpoIIQ and SpoIIAA-AH are produced in the mother cell under σE control and are normally required for activation of σG in the forespore by σE in the mother cell. Possibly these two proteins form a channel linking the two compartments of the Bacillus sporangium (Camp and Losick, 2008).
Meisner et al., 2008 showed that SpoIIIAH is targeted to the forespore membrane. Both SpoIIIAH (218 aas) and SpoIIQ (283 aas) have single N-terminal TMSs with the bulk of the protein on the outside. They have affinity for each other in their extracytoplasmic domains. Consquently they may abut each other to form a trans 2-membrane pore, allowing the mother cell to feed and signal to the forespore. The pore appears to be large enough to transport proteins.
SpoIIIAH is similar to YscJ/FliF of the type III and flagellar systems (TC#3.A.6) of enteric bacteria, and like them, forms ring structures (Meisner et al., 2008). Camp and Losick, 2009 have suggested that the channel formed by SpoIIQ and SpoIIIAH is like a 'feeding tube', like a gap junction of animal cells (see also Kroos, 2009).
Meisner et al. (2012) reported a 2.8-Å resolution structure of a complex of SpoIIQ and SpoIIIAH. SpoIIIAH folds into the ring-building structural motif, and modeling shows that the structure of the SpoIIQ-SpoIIIAH complex is compatible with forming a symmetrical oligomer that is similar to those in type III protein secretion systems (3.A.6). The inner diameters of the two most likely ring models are large enough to accommodate several copies of other integral membrane proteins. SpoIIQ contains a LytM domain, which is found in metalloendopeptidases, but lacks residues important for metalloprotease activity. Other LytM domains appear to be involved in protein-protein interactions. Meisner et al. (2012) found that the LytM domain of SpoIIQ contains an accessory region that interacts with SpoIIIAH.