Yeast Endosulfines Control Entry into Quiescence and Chronological Life Span by Inhibiting Protein Phosphatase 2A

Abstract

The TORC1 and PKA protein kinases are central elements of signaling networks that regulate eukaryotic cell proliferation in response to growth factors and/or nutrients. In yeast, attenuation of signaling by these kinases following nitrogen and/or carbon limitation activates the protein kinase Rim15, which orchestrates the initiation of a reversible cellular quiescence program to ensure normal chronological life span. The molecular elements linking Rim15 to distal readouts including the expression of Msn2/4- and Gis1-dependent genes involve the endosulfines Igo1/2. Here, we show that Rim15, analogous to the greatwall kinase in Xenopus, phosphorylates endosulfines to directly inhibit the Cdc55-protein phosphatase 2A (PP2A(Cdc55)). Inhibition of PP2A(Cdc55) preserves Gis1 in a phosphorylated state and consequently promotes its recruitment to and activation of transcription from promoters of specific nutrient-regulated genes. These results close a gap in our perception of and delineate a role for PP2A(Cdc55) in TORC1-/PKA-mediated regulation of quiescence and chronological life span.

Rim15‐dependent activation of Hsf1 and Msn2/4 transcription factors by direct phosphorylation in Saccharomyces cerevisiae

“…Although Msn2/4 and Gis1 have redundant functions by sharing many common targets, our study has revealed that Msn2, but not Gis1, is a direct substrate of Rim15. The kinase activity of Rim15 has been shown to be necessary for the Gis1-dependent transcription [20], but Rim15 may contribute to the activation of Gis1 through indirect pathway involving Igo1/2 and PP2A CDC55 [16]. Furthermore, we have shown that Rim15 can activate Hsf1 by direct phosphorylation.…”

“…In agreement with the fact that Rim15 is an upstream regulator of Igo1/2, the triple mutant rim15Digo1Digo2D and rim15D showed similar levels of BTN2 and HSP26 expression, which were slightly lower than those in igo1Digo2D. Since Gis1 activity has been shown to be regulated by Igo1/2 via PP2A CDC55 [16], we checked whether Gis1 is involved in BTN2 mRNA expression. However, glucose starvation-dependent induction of BTN2 was not affected by the lack of GIS1 (Fig 3B).…”

“…Upon nutrient starvation, active Yak1 and Rim15 in the nucleus activate Hsf1 and Msn2/4 by direct phosphorylation (white circle). Rim15-dependent phosphorylation of Igo1/2 inhibits PP2A CDC55 , thus activating Gis1 which is a substrate of PP2A CDC55 [16]. In addition, the Igo1/2-PP2A CDC55 signaling pathway prevents 5 0 -3 0 degradation of the induced transcripts [15].…”

“…Igo1 and Igo2, which are phosphorylated by Rim15, antagonize decapping of specific nutrient-regulated mRNAs, thus preventing their 5 0 -3 0 degradation [14,15]. Recently, it has been shown that the phosphorylated form of Igo1 binds to and inhibits Cdc55-protein phosphatase 2A (PP2A CDC55 ), and Gis1 was identified as one of the PP2A CDC55 targets [16]. Therefore, Rim15 indirectly activates Gis1 by preventing its dephosphorylation.…”

PP 1 inactivates Greatwall to release PP 2A‐B55 from mitotic confinement

“…So identification of the PP1 regulatory subunit(s) for Gwl inactivation is very important for cell cycle control. This kinase-phosphatase balancing mechanism composed of Gwl, ARPP/ENSA, PP2A-B55 and PP1 is probably broadly utilized as a functional unit, although a partner kinase, corresponding to CDK, might be different in each case, because PP2A-B55 plays versatile roles in many other biological phenomena, where mitotic CDK not believed to be operating (for example, in the nutrient response in budding yeast [10]). We see this as a useful plug-in module whenever switch-like changes in phosphorylation states are required in Nature.…”