Activation of the proprotein transcription factor pro-sigmaE is associated with its progression through three patterns of subcellular localization during sporulation in Bacillus subtilis.

The activity of the sporulation transcription factor sigmaE in Bacillus subtilis is governed by an intercellular signal transduction pathway that controls the conversion of the inactive proprotein pro-sigmaE to the mature and active form of the factor. Here I use immunofluorescence microscopy to show that the activation of the proprotein is associated with its progression through three patterns of subcellular localization. In the predivisional sporangium, pro-sigmaE was found to be associated with the cytoplasmic membrane. Next, at the stage of asymmetric division, pro-sigmaE accumulated at the sporulation septum. Finally, after processing, mature sigmaE was found to be distributed throughout the mother cell cytoplasm. The results of subcellular fractionation and sedimentation in density gradients of extracts prepared from postdivisional sporangia confirmed that pro-sigmaE was chiefly present in the membrane fraction and that sigmaE was predominantly cytoplasmic, findings that suggest that the pro-amino acid sequence is responsible for the sequestration of pro-sigmaE to the membrane. The results of chemical cross-linking experiments showed that pro-sigmaE was present in a complex with its putative processing protein, SpoIIGA, or with a protein that depended on SpoIIGA. The membrane association of pro-sigmaE was, however, independent of SpoIIGA and other proteins specific to B. subtilis. Likewise, accumulation of pro-sigmaE at the septum did not depend on its interaction with SpoIIGA. Sequestration of pro-sigmaE to the membrane might serve to facilitate its interaction with SpoIIGA and may be important for preventing its premature association with core RNA polymerase. The implications of these findings for the compartmentalization of sigmaE are discussed.