FISSION YEAST ORB6, A SER THR PROTEIN-KINASE RELATED TO MAMMALIAN RHO-KINASE AND MYOTONIC-DYSTROPHY KINASE, IS REQUIRED FOR MAINTENANCE OF CELL POLARITY AND COORDINATES CELL MORPHOGENESIS WITH THE CELL-CYCLE/

The molecular mechanisms that coordinate cell morphogenesis with the c ell cycle remain largely unknown. We have investigated this process in fission yeast where changes in polarized cell growth are coupled with cell cycle progression, The orb6 gene is required during interphase t o maintain cell polarity and encodes a serine/threonine protein kinase , belonging to the myotonic dystrophy kinase/ cot1/warts family. A dec rease in Orb6 protein levels leads to loss of polarized cell shape and to mitotic advance, whereas an increase in Orb6 levels maintains pola rized growth and delays mitosis by affecting the p34(cdc2) mitotic kin ase, Thus the Orb6 protein kinase coordinates maintenance of cell pola rity during interphase with the onset of mitosis, orb6 interacts genet ically with orb2, which encodes the Pak1/Shk1 protein kinase, a compon ent of the Ras1 and Cdc42-dependent signaling pathway. Our results sug gest that Orb6 may act downstream of Pak1/Shk1, forming part of a path way coordinating cell morphogenesis with progression through the cell cycle.