Required during sporulation for activation of sigma factor SpoIIIG/SigG after engulfment is completed in the prespore. Overexpression in the absence of SpoIIIJ is synthetically lethal. Interacts with SpoIIIJ and YqjG. Signal sequence cleavage facilitates function; replacing the signal with the first transmembrane region of SpoIVFB decreases sporulation 7-fold, whereas replacing it with the SleB signal sequence does not affect sporulation. 4000-fold reduction in sporulation efficiency.