Involved in forespore engulfment. 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 SpoIIQ. Localizes to the engulfing septal membranes during spore formation. Specifically expressed in the mother cell during sporulation under the control of the sigma-E factor.