Structure and dynamics of a compact state of a multidomain protein, the mercuric ion reductase.

The functional efficacy of colocalized, linked protein domains is dependent on linker flexibility and system compaction. However, the detailed characterization of these properties in aqueous solution presents an enduring challenge. Here, we employ a novel, to our knowledge, combination of complementary techniques, including small-angle neutron scattering, neutron spin-echo spectroscopy, and all-atom molecular dynamics and coarse-grained simulation, to identify and characterize in detail the structure and dynamics of a compact form of mercuric ion reductase (MerA), an enzyme central to bacterial mercury resistance. MerA possesses metallochaperone-like N-terminal domains (NmerA) tethered to its catalytic core domain by linkers. The NmerA domains are found to interact principally through electrostatic interactions with the core, leashed by the linkers so as to subdiffuse on the surface over an area close to the core C-terminal Hg(II)-binding cysteines. How this compact, dynamical arrangement may facilitate delivery of Hg(II) from NmerA to the core domain is discussed.

Structural characterization of intramolecular Hg(2+) transfer between flexibly linked domains of mercuric ion reductase.

The enzyme mercuric ion reductase MerA is the central component of bacterial mercury resistance encoded by the mer operon. Many MerA proteins possess metallochaperone-like N-terminal domains (NmerA) that can transfer Hg(2+) to the catalytic core domain (Core) for reduction to Hg(0). These domains are tethered to the homodimeric Core by ~30-residue linkers that are susceptible to proteolysis, the latter of which has prevented characterization of the interactions of NmerA and the Core in the full-length protein. Here, we report purification of homogeneous full-length MerA from the Tn21 mer operon using a fusion protein construct and combine small-angle X-ray scattering and small-angle neutron scattering with molecular dynamics simulation to characterize the structures of full-length wild-type and mutant MerA proteins that mimic the system before and during handoff of Hg(2+) from NmerA to the Core. The radii of gyration, distance distribution functions, and Kratky plots derived from the small-angle X-ray scattering data are consistent with full-length MerA adopting elongated conformations as a result of flexibility in the linkers to the NmerA domains. The scattering profiles are best reproduced using an ensemble of linker conformations. This flexible attachment of NmerA may facilitate fast and efficient removal of Hg(2+) from diverse protein substrates. Using a specific mutant of MerA allowed the formation of a metal-mediated interaction between NmerA and the Core and the determination of the position and relative orientation of NmerA to the Core during Hg(2+) handoff.

Antibody array-generated profiles of cytokine release from THP-1 leukemic monocytes exposed to different amphotericin B formulations.

Cytokine antibody arrays were used to establish the profiles of cytokine release from THP-1 monocytes exposed to different amphotericin B (AMB) drug delivery systems. Fungizone (FZ) and Amphotec (ABCD) caused the release of significantly more inflammatory molecules and the release of inflammatory molecules at higher levels than either AmBisome (L-AMB) or Abelcet (ABLC) after 6 h of treatment. Specifically, tumor necrosis factor alpha (TNF-alpha), interleukin-8 (IL-8), GRO-(alphabetagamma), monocyte chemoattractant protein-1 (MCP-1), RANTES, IL-10, and IL-6 were detected and semiquantified with a chemiluminscence imaging system. TNF-alpha, IL-8, and MCP-1 were the most predominant; however, little if any TNF-alpha was present in ABLC- or L-AMB-treated cultures. The TNF- alpha and IL-8 levels determined by quantitative enzyme-linked immunosorbent assay correlated with the relative cytokine levels measured by using the antibody arrays. Although the viabilities of THP-l monocytes in all AMB-treated cultures were similar by trypan blue exclusion, the amount of lactic dehydrogenase released was significantly larger in FZ- and ABCD-treated cultures than in L-AMB- and ABLC-treated cultures, indicating more membrane perturbations with those formulations. Membrane cation channel formation was also measured in model cholesterol-containing large unilamellar vesicles to directly assess the ion channel formation ability of the system. Only FZ and ABCD induced significant ion currents at concentrations less than 1.5 x 10(-5) M. These results may help provide rationales for the immediate cytokine-mediated side effects observed with FZ and ABCD and the reduced side effects observed with L-AMB and ABLC.