Structural and functional characterization of the N-terminal domain of the yeast Mg2+ channel Mrs2.

Mg(2+) translocation across cellular membranes is crucial for a myriad of physiological processes. Eukaryotic Mrs2 transporters are distantly related to the major bacterial Mg(2+) transporter CorA, the structure of which displays a bundle of giant α-helices forming a long pore that extends beyond the membrane before widening into a funnel-shaped cytosolic domain. Here, a functional and structural analysis of the regulatory domain of the eukaryotic Mg(2+) channel Mrs2 from the yeast inner mitochondrial membrane is presented using crystallography, genetics, biochemistry and fluorescence spectroscopy. Surprisingly, the fold of the Mrs2 regulatory domain bears notable differences compared with the related bacterial channel CorA. Nevertheless, structural analysis showed that analogous residues form functionally critical sites, notably the hydrophobic gate and the Mg(2+)-sensing site. Validation of candidate residues was performed by functional studies of mutants in isolated yeast mitochondria. Measurements of the Mg(2+) influx into mitochondria confirmed the involvement of Met309 as the major gating residue in Mrs2, corresponding to Met291 in CorA.

Novel structural insights into F-actin-binding and novel functions of calponin homology domains.

Tandem calponin homology (CH) domains are well-known actin filaments (F-actin) binding motifs. There has been a continuous debate about the details of CH domain-actin interaction, mainly because atomic level structures of F-actin are not available. A recent electron microscopy study has considerably advanced our structural understanding of CH domain:F-actin complex. On the contrary, it has recently also been shown that CH domains can bind other macromolecular systems: two CH domains from separate polypeptides Ncd80, Nuf2 can form a microtubule-binding site, as well as tandem CH domains in the EB1 dimer, while the single C-terminal CH domain of alpha-parvin has been observed to bind to a alpha-helical leucin-aspartate rich motif from paxillin.

The crystal structure of the actin binding domain from alpha-actinin in its closed conformation: structural insight into phospholipid regulation of alpha-actinin.

Alpha-actinin is the major F-actin crosslinking protein in both muscle and non-muscle cells. We report the crystal structure of the actin binding domain of human muscle alpha-actinin-3, which is formed by two consecutive calponin homology domains arranged in a "closed" conformation. Structural studies and available biochemical data on actin binding domains suggest that two calponin homology domains come in a closed conformation in the native apo-form, and that conformational changes involving the relative orientation of the two calponin homology domains are required for efficient binding to actin filaments. The actin binding activity of muscle isoforms is supposed to be regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), which binds to the second calponin homology domain. On the basis of structural analysis we propose a distinct binding site for PtdIns(4,5)P2, where the fatty acid moiety would be oriented in a direction that allows it to interact with the linker sequence between the actin binding domain and the first spectrin-like repeat, regulating thereby the binding of the C-terminal calmodulin-like domain to this linker.

Organization of an efficient carbonic anhydrase: implications for the mechanism based on structure-function studies of a T199P/C206S mutant.

Substitution of Pro for Thr199 in the active site of human carbonic anhydrase II (HCA II)(1) reduces its catalytic efficiency about 3000-fold. X-ray crystallographic structures of the T199P/C206S variant have been determined in complex with the substrate bicarbonate and with the inhibitors thiocyanate and beta-mercaptoethanol. The latter molecule is normally not an inhibitor of wild-type HCA II. All three ligands display novel binding interactions to the T199P/C206S mutant. The beta-mercaptoethanol molecule binds in the active site area with its sulfur atom tetrahedrally coordinated to the zinc ion. Thiocyanate binds tetrahedrally coordinated to the zinc ion in T199P/C206S, in contrast to its pentacoordinated binding to the zinc ion in wild-type HCA II. Bicarbonate binds to the mutant with two of its oxygens at the positions of the zinc water (Wat263) and Wat318 in wild-type HCA II. The environment of this area is more hydrophilic than the normal bicarbonate-binding site of HCA II situated in the hydrophobic part of the cavity normally occupied by the so-called deep water (Wat338). The observation of a new binding site for bicarbonate has implications for understanding the mechanism by which the main-chain amino group of Thr199 acquired an important role for orientation of the substrate during the evolution of the enzyme.