Figure 4. The basic surface area in CHAD domains provides a binding platform for polyPs.
(A) Sulfate ions (in bonds representation) originating from the crystallization buffer are bound to the basic surface area in CtCHAD. Shown are front and back views of CtCHAD as combined ribbon diagram and molecular surface. An electrostatic potential calculated in APBS was mapped onto the molecular surfaces in Pymol. (B) Overview of the polyP complex structure, obtained by crystallization of CtCHAD in the presence of 5 mM polyP (average length ∼7 Pi units). A polyP 9-mer and a tripolyphosphate moiety could be modeled (in bonds representation), with the polyP 9-mer occupying the central pore and extending to both sides. The dashed line indicates the approximate position of several peaks in the Fo-Fc difference electron density map, which could not be modeled with confidence. (C) Structural superposition of the sulfate ion– and polyP-bound CtCHAD structures (r.m.s.d. is ∼0.5 Å comparing 304 corresponding Cα atoms) reveals that the sulfate ions (in bonds representation, in gray) mimic the position of Pi units in the polyP 9-mer (in orange-red) in the CtCHAD-polyP complex.