No abstract

The crystal structure of a thermostable endo-1,5-alpha-L-arabinanase, ABN-TS, from Bacillus thermodenitrificans TS-3 was determined at 1.9 A to an R-factor of 18.3% and an R-free-factor of 22.5%. The enzyme molecule has a five-bladed beta-propeller fold. The substrate-binding cleft formed across one face of the propeller is open on both sides to allow random binding of several sugar units in the polymeric substrate arabinan. The beta-propeller fold is stabilized through a ring closure. ABN-TS exhibits a new closure-mode involving residues in the N-terminal region: Phe7 to Gly21 exhibit hydrogen bonds and hydrophobic interactions with the first and last blades, and Phe4 links the second and third blades through a hydrogen bond and an aromatic stacking interaction, respectively. The role of the N-terminal region in the thermostability was confirmed with a mutant lacking 16 amino acid residues from the N-terminus of ABN-TS.

Casein kinase 2 (CK2) is a serine/threonine kinase that functions as a heterotetramer composed of two catalytic subunits (CK2alpha1 or CK2alpha2) and two regulatory subunits (CK2beta). The two isozymes CK2alpha1 and CK2alpha2 play distinguishable roles in healthy subjects and in patients with diseases such as cancer, respectively. In order to develop novel CK2alpha1-selective inhibitors, the crystal structure of human CK2alpha2 (hCK2alpha2) complexed with a potent CK2alpha inhibitor which binds to the active site of hCK2alpha2 was determined and compared with that of human CK2alpha1. While the two isozymes exhibited a high similarity with regard to the active site, the largest structural difference between the isoforms occurred in the beta4-beta5 loop responsible for the CK2alpha-CK2beta interface. The top of the N-terminal segment interacted with the beta4-beta5 loop via a hydrogen bond in hCK2alpha2 but not in hCK2alpha1. Thus, the CK2alpha-CK2beta interface is a likely target candidate for the production of selective CK2alpha1 inhibitors.

The Ser/Thr kinase CK2 consists of two catalytic subunits (CK2 ) and a dimer of the regulatory subunits (CK2 ), and is a ubiquitous enzyme that regulates growth, proliferation and the survival of cells. CK2 is a remarkable drug target for potentially treating a wide variety of tumours and glomerulonephritis. The purified CK2 protein was crystallized using ethylene glycol as a precipitant. The crystal structure of CK2 with 21 loci of alternative conformations, including a niacin, 19 ethylene glycols and 346 waters, was determined at 1.06 Å resolution to an R work of 14.0% (R free = 16.5%). The alternative ensemble in the internal hydrophobic core underpins the plasticity of the D-helix responsible for the regulation of ATP/GTP binding. The clear density map indicates that a niacin molecule, contained in the Escherichia coli culture medium, binds to the ATP binding site. An ethylene glycol molecule binds in the hydrophobic pocket lateral to the D-helix forming the rim of the active site. The other ethylene glycol molecules occupy physiologically significant sites, including the CK2 binding interface and substrate binding site, as well as the gap in the crystal packing. Together with water molecules in the active site, these structural insights should facilitate drug discovery.