The bacterium Bacillus subtilis undergoes endospore formation in response to starvation. factors play a key role in spatiotemporal regulation of gene expression during development. Activation of factors is coordinated by signal transduction between the forespore and the mother cell. ^E is produced as pro-^E, which is activated in the mother cell by cleavage in response to a signal from the forespore. We report that expression of SpoIIR, a putative signaling protein normally made in the forespore, and SpoIIGA, a putative protease, is necessary and sufficient for accurate, rapid, and abundant processing of pro-^E to ^E in Escherichia coli. Modeling and mutational analyses provide evidence that SpoIIGA is a novel type of aspartic protease whose C-terminal half forms a dimer similar to the human immunodeficiency virus type 1 (HIV-1) protease. Previous studies suggest that SpoIIGA’s N-terminal half is membrane-embedded. We found that SpoIIGA expressed in E. coli is membrane-associated and that after detergent treatment SpoIIGA was self-associated. Also, SpoIIGA interacts with SpoIIR. The results support a model in which SpoIIGA forms inactive dimers or oligomers, and interaction of SpoIIR with SpoIIGA’s N-terminal domain on one side of a membrane causes a conformational change that allows formation of active aspartic protease dimer in the C-terminal domain on the other side of the membrane, where it cleaves pro-^E.