PAS kinase: An evolutionarily conserved PAS domain-regulated serine/threonine kinase

PAS domains regulate the function of many intracellular signaling pathways in response to both extrinsic and intrinsic stimuli. PAS domain-regulated h istidine kinases are common in prokaryotes and control a wide range of fund amental physiological processes. Similarly regulated kinases are rare in eu karyotes and are to date completely absent in mammals. PAS kinase (PASK) is an evolutionarily conserved gene product present in yeast, flies, and mamm als. The amino acid sequence of PASK specifies two PAS domains followed by a canonical serine/threonine kinase domain, indicating that it might repres ent the first mammalian PAS-regulated protein kinase. We present evidence t hat the activity of PASK is regulated by two mechanisms. Autophosphorylatio n at two threonine residues located within the activation loop significantl y increases catalytic activity. We further demonstrate that the N-terminal PAS domain is a cis regulator of PASK catalytic activity. When the PAS doma in-containing region is removed, enzyme activity is significantly increased , and supplementation of the purified PAS-A domain in trans selectively inh ibits PASK catalytic activity. These studies define a eukaryotic signaling pathway suitable for studies of PAS domains in a purified in vitro setting.