Chapter 7 : Protein Targeting during Bacillus subtilis Sporulation

Morphological and protein asymmetry during early stages in sporulation. A. Following entry into sporulation (i), cells establish an asymmetric septum (ii), dividing the sporangium into two unequally sized compartments, the forespore and the mother cell. During engulfment, the septum begins to curve (iii) and continues to curve (iv) until it is attached to the mother cell by only a small patch. Finally, the forespore pinches off from the mother cell (v) and forms a membrane-bounded compartment containing a thick layer of peptidoglycan (gray). Shown are fluorescent microscopy images using the membrane stain FM4-64 (left) and schematic cartoons (right). B. FtsZ rings (Z rings; black) are located at mid-cell during growth (left), but upon entry into sporulation (right), are seen initially in a bipolar pattern and eventually in a unipolar pattern before formation of the asymmetric septum. C. SpoIIE (gray) initially forms “E-rings” that are seen near the bipolar Z rings (left) but following formation of the polar septum (right), SpoIIE is seen on both the mother cell and forespore faces, with apparent enrichment on the forespore face.

Localization of septal proteins early in sporulation. A. The SpoIIIE DNA translocase (red) localizes to the asymmetric septum (i) because of the presence of DNA (blue) and mediates DNA pumping into the forespore (ii). Following completion of septation, SpoIIIE is found in the forespore membrane at the last point of contact with the mother cell (iii). B. The pro-σE processing enzyme SpoIIGA (yellow) initially localizes to the sites of incipient septum formation (i) and then to the mother cell face of the asymmetric septum (ii). The SpoIIR signaling protein (green) is made in the forespore and crosses the forespore membrane where it presumably interacts with and activates SpoIIGA, although it is also seen in the forespore following completion of septation. C. SpoIIB (aqua) initially colocalizes with FtsZ (orange) during the process of Z-ring constriction and remains in the polar septum following completion of septation. The SpoIIM (light blue), SpoIIP (dark blue), and SpoIID (purple) proteins proceed to localize to the now curved asymmetric septum.

Localization of septal proteins later in sporulation. A. Expression of SpoIIIAH (lime) is under control of σE and it is initially found in all of the mother cell membrane (i). Expression of SpoIIQ (purple) is under control of σF and it is initially found in the forespore septal membrane. Interaction of SpoIIQ and SpoIIIAH in the septal intermembrane space leads to localization of SpoIIIAH to the septum (ii), and this interaction continues until late in engulfment (iii). B. Initially, contact between SpoIIQ in the forespore membrane and SpoIIIAH in the mother cell membrane is prevented because of the presence of peptidoglycan. However, removal of this layer allows contact between the two proteins presumably through “extracellular” domains, resulting in the enrichment of SpoIIIAH at the septum. C. SpoIVFB (red) is initially observed in all mother cell membranes, but it eventually is “captured” by SpoIVFA (orange) and becomes enriched at the forespore in a complex with the SpoIVFB and BofA. D. SpoIVFA interacts with a number of proteins in the forespore outer membrane including SpoIID (green), SpoIIP (dark blue), SpoIIM (light blue), SpoIIIAH (lime), and SpoIIQ (purple).

Localization of proteins involved in spore coat and cortex assembly. A. SpoVM (green) has an intrinsic affinity for the forespore. B. SpoVD (red) and SpoVE (orange) form a complex at the outer forespore membrane. C. In the absence of SpoVE, SpoVD is found throughout the mother cell membrane.