GLC7 encodes the catalytic subunit of type 1 protein serine/threonine phosphatase (PP1) in the yeast Saccharomyces cerevisiae. Here we have characterized the temperature-sensitive glc7-10 allele, which displays aberrant bud morphology and an abnormal actin cytoskeleton at the restrictive temperature. At 37 degrees C glc7-10 strains accumulated a high proportion of budded cells with an unmigrated nucleus, duplicated spindle pole bodies, a short spindle, delocalized cortical actin and 2C DNA content, indicating a cell cycle block prior to the metaphase to anaphase transition. glc7-10 was suppressed by growth on high osmolarity medium and exhibited temperature-sensitive cell lysis upon hypo-osmotic stress. Pkc1p, the yeast protein kinase C homolog which is thought to regulate the Mpk1p MAP kinase pathway involved in cell wall remodelling and polarized cell growth, was found to act as a dosage suppressor of glc7-10. Although neither activation of BCK1 (MEKK) by the dominant BCK1-20 mutation nor increased dosage of MKK1 (MEK) or MPK1 (MAP kinase) mimicked PKC1 as a glc7-10 dosage suppressor, extra copies of genes encoding upstream components of the Pkc1p pathway such as ROM2, RHO2, HCS77/WSC1/SLG1 and MID2 also suppressed glc7-10 effectively. Conversely, mpk1delta glc7-10 and bck1delta glc7-10 double mutants displayed a synthetic cell lysis defect compared with each single mutant and glc7-10 was hypersensitive to reduced PKC1 function, displaying highly aberrant morphologies and inviability even at the normally permissive temperature of 26 degrees C. Dephosphorylation by PP1 therefore functions positively to promote cell integrity, bud morphology and polarization of the actin cytoskeleton and glc7-10 cells require higher levels of Pkc1p activity to sustain these functions.