A protein phosphorylation module patterns the Bacillus subtilis spore outer coat.

Carolina Freitas,Jarnaja Plannic,Ezio Ricca,Loredana Baccigalupi,Rachele Isticato,Rita Zilhão,Alexander K W Elsholz,Adriano O Henriques,Mónica Serrano,Assunta Pelosi,Richard Losick

doi:10.1111/mmi.14562

Abstract

Assembly of the Bacillus subtilis spore coat involves over 80 proteins which self‐organize into a basal layer, a lamellar inner coat, a striated electrodense outer coat and a more external crust. CotB is an abundant component of the outer coat. The C‐terminal moiety of CotB, SKRB, formed by serine‐rich repeats, is polyphosphorylated by the Ser/Thr kinase CotH. We show that another coat protein, CotG, with a central serine‐repeat region, SKRG, interacts with the C‐terminal moiety of CotB and promotes its phosphorylation by CotH in vivo and in a heterologous system. CotG itself is phosphorylated by CotH but phosphorylation is enhanced in the absence of CotB. Spores of a strain producing an inactive form of CotH, like those formed by a cotG deletion mutant, lack the pattern of electrondense outer coat striations, but retain the crust. In contrast, deletion of the SKRB region, has no major impact on outer coat structure. Thus, phosphorylation of CotG by CotH is a key factor establishing the structure of the outer coat. The presence of the cotB/cotH/cotG cluster in several species closely related to B. subtilis hints at the importance of this protein phosphorylation module in the morphogenesis of the spore surface layers.

Highlights

Bacterial endospores are a dormant cell type formed by a diverse group of bacteria within the Firmicutes phylum
While deletion of the SKRB region has no impact on the structure of the coat, we show that mutants lacking active CotH or CotG form an amorphous outer coat which is still delimited at its outer edge by the crust
The Ser residues within SKRB and SKRG, but much less so outside this region, show a high probability of being targets for phosphorylation (Fig. S1B)

Summary

Introduction

Bacterial endospores (spores for simplicity) are a dormant cell type formed by a diverse group of bacteria within the Firmicutes phylum. Sporulation occurs within a sporangium formed by a larger mother cell, and a smaller forespore, or future spore. In Bacillus subtilis, the outermost spore layer is the coat, a proteinbound organelle that protects mature spores and mediates their interaction with abiotic and biotic surfaces and with germinants (Driks & Eichenberger, 2016, Henriques & Moran, 2007, McKenney et al, 2013). The coat comprises a basal layer, a lamellar inner coat, a striated elecontrondense outer coat and a more external crust (Driks & Eichenberger, 2016, Henriques & Moran, 2007, McKenney et al, 2013).

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Results

Conclusion