MKH1, A MEK KINASE REQUIRED FOR CELL-WALL INTEGRITY AND PROPER RESPONSE TO OSMOTIC AND TEMPERATURE STRESS IN SCHIZOSACCHAROMYCES-POMBE

We have identified a Schizosaccharomyces gene, mkh1, that encodes a ME K kinase (MEKK) homolog, The coding region of mkh1 is contained within a single exon encoding a 1,116-amino-acid protein. The putative catal ytic domain of Mkh1 is 54% identical to the catalytic domain of S. cer evisiae Bck1, the most closely related protein. Deletion of mkh1 did n ot significantly affect cell growth or division under standard conditi ons, However, mkh1 Delta cell growth was inhibited by high KCl or NaCl concentrations. mkh1 Delta cells required a longer time to reenter th e cell cycle after prolonged stationary-phase arrest, Also, mkh1 Delta cells exhibited a round cell shape, while overexpression of Mkh1 resu lted in an elongated cell shape, mkh1 Delta cells exhibited a more dra matic phenotype when grown in nutrient-limiting conditions at high tem perature or in hyperosmotic medium. In such conditions, completion of cytokinesis was inhibited, resulting in the growth of pseudohyphal fil aments with. multiple sept-a and nuclei, Also, mkh1 Delta cells were h ypersensitive to beta-glucanase treatment, Together these results sugg est that Mkh1 regulates cell morphology, cell wall integrity, salt res istance, cell cycle reentry from stationary-phase arrest, and filament ous growth in response to stress, These phenotypes are essentially ide ntical to those exhibited by cells lacking Pmk1/Spm1, a recently ident ified mitogen-activated protein kinase, Our evidence suggests that Pmk 1/Spm1 acts downstream from Mkh1 in a common pathway. Our results also suggest that Mkh1 and Pck2 act independently to maintain cell wall in tegrity, cell morphology, and sale resistance but act: in opposition t o regulate filamentous growth.