Meta-Biol

• Pathways

PERK (EIF2AK3) is a single-pass transmembrane protein located in the endoplasmic reticulum (ER) membrane such that the N-terminus of PERK is luminal and the C-terminus is cytosolic. PERK is maintained in an inactive form by interaction of its luminal domain with BiP, an ER chaperone. BiP also binds unfolded proteins and so BiP dissociates from PERK when unfolded proteins accumulate in the ER. Dissociated PERK monomers spontaneously form homodimers and the homodimeric form of PERK possesses kinase activity in its cytosolic C-terminal domain. The kinase specifically phosphorylates the translation factor eIF2alpha at Ser52, resulting in an arrest of translation. Thus translation of proteins targeted to the ER is downregulated. The translation arrest also causes depletion of Cyclin D1, a rapidly turned over protein. The depletion of Cyclin D1 in turn causes arrest of the cell cycle in G1 phase.

Cells are subject to external molecular and physical stresses such as foreign molecules that perturb metabolic or signaling processes, and changes in temperature or pH. Cells are also subject to internal molecular stresses such as production of reactive metabolic byproducts. The ability of cells and tissues to modulate molecular processes in response to such stresses is essential to the maintenance of tissue homeostasis (Kultz 2005). Specific stress-related processes annotated here are cellular response to hypoxia, cellular response to heat stress, cellular senescence, HSP90 chaperone cycle for steroid hormone receptors (SHR) in the presence of ligand, response of EIF2AK1 (HRI) to heme deficiency, heme signaling, cellular response to chemical stress, cellular response to starvation, and unfolded protein response.

Individual cells detect and respond to diverse external molecular and physical signals. Appropriate responses to these signals are essential for normal development, maintenance of homeostasis in mature tissues, and effective defensive responses to potentially noxious agents (Kultz 2005). It is convenient, if somewhat arbitrary, to distinguish responses to signals involved in development and homeostasis from ones involved in stress responses, and that classification is followed here, with macroautophagy and responses to metal ions classified as responses to normal external stimuli, while responses to hypoxia, reactive oxygen species, and heat, and the process of cellular senescence are classified as stress responses. Signaling cascades are integral components of all of these response mechanisms but because of their number and diversity, they are grouped in a separate signal transduction superpathway in Reactome.