The crystal structure of a fully active form of human protein kinase CK2 (casein kinase 2) consisting of two C‐terminally truncated catalytic and two regulatory subunits has been determined at 3.1 Å resolution (Protein Data Bank code: 1JWH). In the CK2 complex the regulatory subunits form a stable dimer linking the two catalytic subunits, which make no direct contact with one another. Each catalytic subunit interacts with both regulatory chains, predominantly via an extended C‐terminal tail of the regulatory subunit. The CK2 structure is consistent with its constitutive activity and with a flexible role of the regulatory subunit as a docking partner for various protein kinases. Furthermore it shows an inter‐domain mobility in the catalytic subunit known to be functionally important in protein kinases and detected here for the first time directly within one crystal structure.

• Enhanced Disease Susceptibility1 (EDS1) is an important regulator of plant basal and receptor‐triggered immunity. Arabidopsis EDS1 interacts with two related proteins, Phytoalexin Deficient4 (PAD4) and Senescence Associated Gene101 (SAG101), whose combined activities are essential for defense signaling. The different sizes and intracellular distributions of EDS1–PAD4 and EDS1–SAG101 complexes in Arabidopsis leaf tissues suggest that they perform nonredundant functions. • The nature and biological relevance of EDS1 interactions with PAD4 and SAG101 were explored using yeast three‐hybrid assays, in vitro analysis of recombinant proteins purified from Escherichia coli, and characterization of Arabidopsis transgenic plants expressing an eds1 mutant (eds1^L262P) protein which no longer binds PAD4 but retains interaction with SAG101. • EDS1 forms molecularly distinct complexes with PAD4 or SAG101 without additional plant factors. Loss of interaction with EDS1 reduces PAD4 post‐transcriptional accumulation, consistent with the EDS1 physical association stabilizing PAD4. The dissociated forms of EDS1 and PAD4 are fully competent in signaling receptor‐triggered localized cell death at infection foci. By contrast, an EDS1–PAD4 complex is necessary for basal resistance involving transcriptional up‐regulation of PAD4 itself and mobilization of salicylic acid defenses. • Different EDS1 and PAD4 molecular configurations have distinct and separable functions in the plant innate immune response.

CK2α is the catalytic subunit of protein kinase CK2, an acidophilic and constitutively active eukaryotic Ser/Thr kinase involved in cell proliferation. A crystal structure, at 2.1 Å resolution, of recombinant maize CK2α (rmCK2α) in the presence of ATP and Mg2+, shows the enzyme in an active conformation stabilized by interactions of the N‐terminal region with the activation segment and with a cluster of basic residues known as the substrate recognition site. The close interaction between the N‐terminal region and the activation segment is unique among known protein kinase structures and probably contributes to the constitutively active nature of CK2. The active centre is occupied by a partially disordered ATP molecule with the adenine base attached to a novel binding site of low specificity. This finding explains the observation that CK2, unlike other protein kinases, can use both ATP and GTP as phosphorylating agents.

High‐resolution crystal structures of α‐l‐arabinofuranosidase from Geobacillus stearothermophilus T‐6, a family 51 glycosidase, are described. The enzyme is a hexamer, and each monomer is organized into two domains: a (β/α)8‐barrel and a 12‐stranded β sandwich with jelly‐roll topology. The structures of the Michaelis complexes with natural and synthetic substrates, and of the transient covalent arabinofuranosyl—enzyme intermediate represent two stable states in the double displacement mechanism, and allow thorough examination of the catalytic mechanism. The arabinofuranose sugar is tightly bound and distorted by an extensive network of hydrogen bonds. The two catalytic residues are 4.7 Å apart, and together with other conserved residues contribute to the stabilization of the oxocarbenium ion‐like transition state via charge delocalization and specific protein—substrate interactions. The enzyme is an anti ‐protonator, and a 1.7 Å electrophilic migration of the anomeric carbon takes place during the hydrolysis.