The Bacillus subtilis SpoIVB protein is a critical component of the intercompartmental signal-transduction pathway that activates the sigma factor, delta K, in the mother cell of the sporulating cell. SpoIVB, synthesized in the forespore chamber, must act across two layers of phospholipid membrane to facilitate proteolytic processing of inactive pro-delta K to active delta K. We have used a genetic approach to dissect SpoIVB function and found that this protein has two distinct developmental functions. One function is that of intercompartmental signalling of pro-delta K processing. The other role is essential to spore formation and is illustrated by mutations of SpoIVB which allow cell-cell signalling of pro-delta K processing but prevent the formation of viable spores. Using localized and site-specific mutagenesis we have identified a functional domain of SpoIVB that is involved in its non-signalling role.