Evidence for concerted functions of protein kinase A, phospholipase B1, and the arrestin‐related protein, Aks1, in adaptive responses to KCl and rotenone stresses in fission yeast

We have isolated a previously uncharacterized gene, aks1 , as a multi‐copy suppressor of KCl‐sensitive growth defects of protein kinase A ( pka1 Δ) and phospholipase B1 ( plb1 Δ) null mutants of the fission yeast, Schizosaccharomyces pombe . The predicted Aks1 protein is related to arrestin‐like proteins in budding yeast and other fungi, which are believed to function as adaptors for Nedd4‐related ubiquitin ligases. Similar to pka1 Δ and plb1 Δ mutants, S. pombe aks1 Δ mutants are hypersensitive to KCl stress and the toxin rotenone. Unlike plb1 Δ and pka1 Δ mutants, aks1 Δ mutants are cold sensitive for growth and exhibit defects in cell morphology. aks1 Δ pka1 Δ and pka1 Δ plb1 Δ double mutants exhibit KCl‐ and rotenone‐hypersensitive growth defects that are greater in severity than the corresponding single mutants. At the same time, Pka1 hyperactivation suppresses KCl and rotenone‐hypersensitive growth defects of plb1Δ and aks1Δ mutants and the cold sensitive growth defect of the aks1Δ mutant. Interestingly, both pka1 Δ and aks1 Δ mutants exhibit stress‐induced defects in vacuole size and/or localization that are either similar or distinct, depending on the stress. Aks1 appears to be monoubiquitinated and localized to cytoplasmic dots of as yet undetermined identity in non‐stressed cells. Effects of stresses on Aks1 ubiquitination and localization are being investigated. Our results suggest that Pka1, Aks1, and Plb1 have at least partially concerted functions in adaptive responses to KCl, rotenone, and possibly other stresses in S. pombe .