The heme-regulated eukaryotic initiation factor 2␣ (eIF2␣) kinase (heme-regulated inhibitor (HRI)) is activated by heme deficiency in reticulocytes and plays an important role in translational control in these cells. Previously, HRI was cloned from rabbit reticulocytes and rat brain, but a heme-regulated eIF2␣ kinase activity has only been purified from erythroid cells. In this study, we report the purification of a heme-sensitive eIF2␣ kinase activity from both mouse liver and NIH 3T3 cell extracts. Furthermore, we have cloned and characterized this mouse liver eIF2␣ kinase (mHRI), which exhibits 83 and 94% identities to rabbit and rat HRIs, respectively. Both the purified enzyme and recombinant mHRI exhibited an autokinase and an eIF2␣ kinase activity, and both activities were inhibited in vitro by hemin. In addition, wild-type mHRI, but not the inactive mHRI-K196R mutant, was autophosphorylated in vivo when it was expressed in 293 cells. Quantitation of mHRI mRNA expression in various mouse tissues by reverse transcription-polymerase chain reaction revealed relatively high levels in liver, kidney, and testis. These results provide strong evidence that mHRI is a ubiquitous eIF2␣ kinase of mammalian cells, suggesting that it could play important roles in the translational regulation of nonerythroid tissues.

Polarized growth in filamentous fungi depends on the correct spatial organization of the microtubule (MT) and actin cytoskeleton. In Schizosaccharomyces pombe it was shown that the MT cytoskeleton is required for the delivery of so-called cell end marker proteins, e.g., Tea1 and Tea4, to the cell poles. Subsequently, these markers recruit several proteins required for polarized growth, e.g., a formin, which catalyzes actin cable formation. The latest results suggest that this machinery is conserved from fission yeast to Aspergillus nidulans. Here, we have characterized TeaC, a putative homologue of Tea4. Sequence identity between TeaC and Tea4 is only 12.5%, but they both share an SH3 domain in the N-terminal region. Deletion of teaC affected polarized growth and hyphal directionality. Whereas wild-type hyphae grow straight, hyphae of the mutant grow in a zig-zag way, similar to the hyphae of teaA deletion (tea1) strains. Some small, anucleate compartments were observed. Overexpression of teaC repressed septation and caused abnormal swelling of germinating conidia. In agreement with the two roles in polarized growth and in septation, TeaC localized to hyphal tips and to septa. TeaC interacted with the cell end marker protein TeaA at hyphal tips and with the formin SepA at hyphal tips and at septa.

Controlling aberrant kinase-mediated cellular signaling is a major strategy in cancer therapy; successful protein kinase inhibitors such as Tarceva and Gleevec verify this approach. Specificity of inhibitors for the targeted kinase(s), however, is a crucial factor for therapeutic success. Based on homology modeling, we previously identified four amino acids in the active site of Rho-kinase that likely determine inhibitor specificities observed for Rho-kinase relative to protein kinase A (PKA) (in PKA numbering: T183A, L49I, V123M, and E127D), and a fifth (Q181K) that played a surprising role in PKA-PKB hybrid proteins. We have systematically mutated these residues in PKA to their counterparts in Rho-kinase, individually and in combination. Using four Rho-kinase-specific, one PKA-specific, and one pan-kinase-specific inhibitor, we measured the inhibitor-binding properties of the mutated proteins and identify the roles of individual residues as specificity determinants. Two combined mutant proteins, containing the combination of mutations T183A and L49I, closely mimic Rho-kinase. Kinetic results corroborate the hypothesis that side-chain identities form the major determinants of selectivity. An unexpected result of the analysis is the consistent contribution of the individual mutations by simple factors. Crystal structures of the surrogate kinase inhibitor complexes provide a detailed basis for an understanding of these selectivity determinant residues. The ability to obtain kinetic and structural data from these PKA mutants, combined with their Rho-kinase-like selectivity profiles, make them valuable for use as surrogate kinases for structure-based inhibitor design.Phosphorylation via protein kinases is responsible for a large part of cellular signal transduction and is described as a universal regulatory mechanism (1, 2). Perturbation of kinase-mediated signaling pathways results in a number of diseases, including diabetes, cancer, and inflammation (3, 4). Because most protein kinases reside in the cell in an inactive state and are activated by signal transduction processes, many diseases are triggered by overactivation of protein kinases via mutation, overexpression, or malfunctioning cellular inhibition.The human genome encodes some 518 protein kinases (5) that are notably different in how their catalysis is regulated but share a catalytic domain conserved in sequence and structure (6, 7). The latter consists of 250 -300 amino acids, binds substrate and cosubstrate, and catalyzes the phosphorylation reaction.This catalytic domain, together with less conserved surrounding sites, has been the focus of inhibitor design that has exploited differences in kinase structure and pliability to achieve selectivity. Many drugs that target protein kinases are in clinical trials, and some have already been approved, such as the Abl kinase inhibitor Gleevec for therapy against chronic myelogenous leukemia (8) and Tarceva (erlotinib) against nonsmall cell lung cancer (9). The first protein kinase inhibitor that passed the cli...