Yeast karyopherins Kap123 and Kap95 are related to the function of the cell integrity pathway

The characterization of mutant strains in the gene encoding karyopherin Kap123 has revealed several morphogenetic defects. Inactivation of KAP123 caused alterations in the actin cytoskeleton, resulting in hyperpolarization and resistance to the actin polymerization inhibitor latrunculin B. In fact, the level of actin filaments is increased in kap123 mutant cells. In addition to the defect in actin cytoskeleton, the kap123 mutant cells showed a weakened cell wall, cell lysis and a growth defect in either the presence of sodium dodecyl sulfate or at high temperatures, which is alleviated by osmotic stabilizers. These defects in cell integrity and the actin cytoskeleton suggested a relationship with the protein kinase C (PKC) cell integrity pathway. Slt2, the mitogen-activated protein kinase of the PKC cell integrity pathway, is constitutively activated in the absence of Kap123, which is consistent with the existence of cell integrity defects. Analysis of the subcellular localization of nuclear proteins involved in cell wall gene expression indicated that the localization of the Slt2 kinase and the transcription factors Rlm1, Swi6 and Paf1 was not affected by Kap123. Finally, we identified karyopherin Kap95 as the transport factor responsible for the nuclear import of Slt2 and Rlm1.