Involved in forespore engulfment and required for anchoring membrane proteins on the forespore side of the septal membrane. 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. Interacts with SpoIIIAH and SpoIIE. Localizes to the engulfing septal membranes. Specifically expressed in the forespore under the control of the sigma-K factor.