Eicken Christoph, Pennella Mario A, Chen Xiaohua, Koshlap Karl M, VanZile Michael L, Sacchettini James C, Giedroc David P
Department of Biochemistry and Biophysics, Center for Structural Biology, Texas A&M University, College Station, TX 77843-2128, USA.
J Mol Biol. 2003 Oct 31;333(4):683-95. doi: 10.1016/j.jmb.2003.09.007.
The origin of metal ion selectivity by members of the SmtB/ArsR family of bacterial metal-sensing transcriptional repressors and the mechanism of negative allosteric regulation of DNA binding is poorly understood. Here, we report that two homologous zinc sensors, Staphylococcus aureus CzrA and cyanobacterial SmtB, are "winged" helix homodimeric DNA-binding proteins that bind Zn(II) to a pair of tetrahedral, interhelical binding sites, with two ligands derived from the alpha5 helix of one subunit, Asp84 O(delta1) (Asp104 in SmtB), His86 N(delta1) (His106), and two derived from the alpha5 helix of the other, His97' N(delta1) (His117') and His100' N(epsilon2) (Glu120'). Formation of the metal chelate drives a quaternary structural switch mediated by an intersubunit hydrogen-binding network that originates with the non-liganding N(epsilon2) face of His97 in CzrA (His117 in SmtB) that stabilizes a low-affinity, DNA-binding conformation. The structure of the Zn(1) SmtB homodimer shows that both metal-binding sites of the dimer must be occupied for the quaternary structural switch to occur. Thus, a critical zinc-ligating histidine residue obligatorily couples formation of the metal-sensing coordination chelate to changes in the conformation and dynamics of the putative DNA-binding helices.
细菌金属感应转录阻遏物SmtB/ArsR家族成员对金属离子的选择性起源以及DNA结合的负变构调节机制目前还知之甚少。在此，我们报道了两种同源锌传感器，金黄色葡萄球菌CzrA和蓝藻SmtB，它们是“带翼”螺旋同源二聚体DNA结合蛋白，将Zn(II)结合到一对四面体的螺旋间结合位点，其中两个配体来自一个亚基的α5螺旋，即Asp84 O(δ1)（SmtB中的Asp104）、His86 N(δ1)（His106），另外两个来自另一个亚基的α5螺旋，即His97' N(δ1)（His117'）和His100' N(ε2)（Glu120'）。金属螯合物的形成驱动了由亚基间氢键网络介导的四级结构转换，该网络起源于CzrA中His97（SmtB中His117）的非配体N(ε2)面，稳定了低亲和力的DNA结合构象。Zn(1) SmtB同源二聚体的结构表明，二聚体的两个金属结合位点都必须被占据，四级结构转换才会发生。因此，一个关键的锌配位组氨酸残基必然将金属感应配位螯合物的形成与假定的DNA结合螺旋的构象和动力学变化联系起来。
