Structure of the RsbX phosphatase involved in the general stress response of Bacillus subtilis

In the general stress response of Bacillus subtilis, which is governed by the sigma factor σ^B, stress signalling is relayed by a cascade of Rsb proteins that regulate σ^B activity. RsbX, a PPM II phosphatase, halts the response by dephos­phorylating the stressosome composed of RsbR and RsbS. The crystal structure of RsbX reveals a reorganization of the catalytic centre, with the second Mn²⁺ ion uniquely coordinated by Gly47 O from the β4–α1 loop instead of a water molecule as in PPM I phosphatases. An extra helical turn of α1 tilts the loop towards the metal-binding site, and the β2–β3 loop swings outwards to accommodate this tilting. The residues critical for this defining feature of the PPM II phosphatases are highly conserved. Formation of the catalytic centre is metal-specific, as crystallization with Mg²⁺ ions resulted in a shift of the β4–α1 loop that led to loss of the second ion. RsbX also lacks the flap subdomain characteristic of PPM I phosphatases. On the basis of a stressosome model, the activity of RsbX towards RsbR-P and RsbS-P may be influenced by the different accessibilities of their phosphorylation sites.