PASKIN, a PAS, AMP-like, serine/threonine kinase highly expressed in the testis.

K. Eckhardt (1), D.M. Katschinski (2), H.H. Marti (3) and R.H. Wenger (1).

Institute of Physiology, University of Zürich (1); Cell Physiology Group, Medical Faculty, Martin-Luther-University Halle (2); Institute of Physiology and Pathophysiology, University of Heidelberg (3).  

The PAS domain is an universal sensory domain found in many proteins capable of sensing changes in light intensity, oxygen partial pressure, redox potentials, voltage and the concentration of certain ligands. While most PAS proteins are known from archaea, bacteria and lower eukarya, relatively few sensor PAS proteins have been reported in mammalian species. We and others recently discovered a novel mammalian PAS protein, termed PASKIN or PASK, which is related to the oxygen sensor FixL from Rhizobium species. PASKIN contains two PAS domains and a serine/threonine kinase domain related to AMP kinases. The yeast PASKIN homologs phosphorylate three translation factors and two enzymes involved in the regulation of glycogen and trehalose synthesis, thereby co-ordinately controlling protein translation and sugar flux. Under stress conditions PASKIN kinase activity results in down-regulation of protein synthesis and carbohydrate storage. In isolated pancreatic islet beta cells high glucose concentrations leads to a rapid increase of PASKIN activity. Under these conditions PASKIN appears to be involved in mediating effects of glucose on pre-pro-insulin gene transcription. To investigate the function of mammalian PASKIN, we knocked out the mouse Paskin gene by homologous recombination in embryonic stem cells. Paskin knock-out mice showed at least under laboratory conditions normal development, growth and reproduction. The targeted integration of a lacZ reporter gene allowed the identification of the cell types expressing mouse PASKIN. Surprisingly, PASKIN expression was detectable almost exclusively in post-meiotic germ cells during spermatogenesis. The testis is an organ where profound differentiation processes take place under very low oxygen concentrations. Spermatogenesis is prone to environmental influences, often resulting in male infertility due to toxic pollutants or thermic stress. The molecular mechanisms of these processes are only marginally understood. In an initial yeast two-hybrid screen for novel proteins interacting with PASKIN, we recently identified an interaction-partner involved in protein metabolism. Currently, we are analysing this protein-protein interaction in more detail in vitro.