Cloning and characterization of an eukaryotic initiation factor-2α kinase from the silkworm, Bombyx mori

Abstract

Eukaryotic initiation factor 2α (eIF-2α) kinases are involved in the translational regulations that occur in response to various types of environmental stress, and play an important role in the cellular defense system operating under unfavorable conditions. The identification of additional eIF-2α kinases and the elucidation of their functions are necessary to understand how different eIF-2α kinases can specifically respond to distinct stimuli. Here, we report a novel eIF-2α kinase, termed BeK, from the silkworm, Bombyx mori. This gene encodes 579 amino acids and contains all 11 catalytic domains of protein-serine/threonine kinases. Most notably, it contains an "Ile-Gln-Met-Xaa-Xaa-Cys" motif, which is highly conserved from yeast to mammalian eIF-2α kinases. BeK does not show any significant homology in the NH₂ terminal regulatory domain, suggesting a distinct regulatory mechanism of this novel eIF-2α kinase. BeK is ubiquitously expressed in the various tissues throughout the final larval stage. Importantly, BeK is activated in Drosophila Schneider cells following heat shock and osmotic stress, and activated-BeK has been shown to phosphorylate an eIF-2α subunit at the Ser⁵⁰ site. However, other forms of stress, such as immune stress, endoplasmic reticulum stress and oxidative stress, cannot significantly elicit BeK activity. Interestingly, the baculovirus gene product, PK2, can inhibit BeK enzymatic activity, suggesting that BeK may be an endogenous target for a viral gene product. Taken together, these data indicate that BeK is a novel eIF-2α kinase involved in the stress response in B. mori.