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. 2021 Apr 21;6(2):e00007-21. doi: 10.1128/mSphere.00007-21

FIG 5.

FIG 5

Successive localization, interactions, and interdependence among CotE, CotY, and ExsY during exosporium formation. (A) (Left) Assembly of the indicated spore structures as observed by TEM. Dotted lines show the new structures found in the present study. (Right) Diagrams showing the temporal sequence of localization of CotE, CotY, and ExsY inferred from our results. (i) CotE (red) forms a cap in the septal region at the onset of engulfment and recruits CotY (blue), which, in turn, recruits ExsY (gray). Once positioned, CotY and ExsY form the basal layer of the cap (mixed gray and blue). The cap remains unchanged until the completion of engulfment. (ii) After the completion of engulfment, the MCP cap is separated from the OFM by the formation of the interspace (IS), and CotE directs the localization of CotY, and therefore of ExsY, as a second cap on the MCD pole (dotted line). (iii) CotE progressively encases the spore starting from one longitudinal side of the forespore (red arrows), guiding the simultaneous encasement of ExsY (gray arrows), which, in turn, is required for the encasement of CotY (blue arrows). After the completion of encasement by CotE, CotY and ExsY cover the noncap region of the forespore as an immature morphogenetic scaffold (dotted line). (iv) After coat formation and insertion of the late-synthesized proteins, CotE, CotY, and ExsY are found in all the regions of the mature exosporium. (B) Formation of the morphogenetic scaffold and its maturation in B. cereus. Based on their roles in B. subtilis, SpoIVA and SpoVID homologs are good candidates for directing the recruitment and encasement, respectively, of CotE and CotE-controlled proteins. CotO, possibly through its interaction with CotE, may participate in the recruitment of CotY and/or ExsY. ExsA, a SafA homologue that controls B. cereus coat protein deposition, is also present in the morphogenetic scaffold. After sporulation completion, the proteins of the morphogenetic scaffold are positioned in the different layers of the mature spore.