Stage III sporulation protein AH (SpoIIIAH) is a protein that is involved in forespore engulfment. It forms a channel with SpoIIIAH that is open on the forespore end and closed (or gated) on the mother cell end. This allows sigma-E-directed gene expression in the mother-cell compartment of the sporangium to trigger the activation of sigma-G forespore-specific gene expression by a pathway of intercellular signaling. This family of proteins is found in bacteria, archaea and eukaryotes and so must have a wider function that in sporulation. Proteins in this family are typically between 174 and 223 amino acids in length.

1: A novel pathway of intercellular signalling in Bacillus subtilis involves a protein with similarity to a component of type III secretion channels. Camp AH, Losick R; Mol Microbiol. 2008;69:402-417. PMID:18485064