A Bacillus subtilis secreted protein with a role in endospore coat assembly and function

Bacterial endospores are encased in a complex protein coat, which confers p rotection against noxious chemicals and influences the germination response . In Bacillus subtilis, over 20 polypeptides are organized into an amorphou s undercoat, a lamellar lightly staining inner structure, and an electron-d ense outer coat. Here we report on the identification of a polypeptide of a bout 30 kDa required for proper coat assembly, which was extracted from spo res of a gerE mutant. The N-terminal sequence of this polypeptide matched t he deduced product of the tasA gene, after removal of a putative 27-residue signal peptide, and TasA was immunologically detected in material extracte d from purified spores. Remarkably, deletion of tasA results in the product ion of asymmetric spores that accumulate misassembled material in one pole and have a greatly expanded undercoat and an altered outer coat structure. Moreover, we found that tasA and gerE mutations act synergistically to decr ease the efficiency of spore germination. We show that tasA is the most dis tal member of a three-gene operon, which also encodes the type I signal pep tidase SipW. Expression of the tasA operon is enhanced 2 h after the onset of sporulation, under the control of sigma(H). When tasA transcription is u ncoupled from sipW expression, a presumptive TasA precursor accumulates, su ggesting that its maturation depends on SipW. Mature TasA is found in super natants of sporulating cultures and intracellularly from 2 h of sporulation onward. We suggest that, at an early stage of sporulation, TasA is secrete d to the septal compartment. Later, after engulfment of the prespore by the mother cell, TasA acts from the septal-proximal pole of the spore membrane s to nucleate the organization of the undercoat region. TasA is the first e xample of a polypeptide involved in coat assembly whose production is not m other cell specific but rather precedes its formation. Our results implicat e secretion as a mechanism to target individual proteins to specific cellul ar locations during the assembly of the bacterial endospore coat.