S6K1-Mediated Disassembly of Mitochondrial URI/PP1γ Complexes Activates a Negative Feedback Program that Counters S6K1 Survival Signaling

Abstract

S6 kinase 1 (S6K1) acts to integrate nutrient and growth factor signals to promote cell growth but also cell survival as a mitochondria-tethered protein kinase that phosphorylates and inactivates the proapoptotic molecule BAD. Here we report that the prefoldin chaperone URI represents a mitochondrial substrate of S6K1. In growth factor-deprived or rapamycin-treated cells, URI forms stable complexes with protein phosphatase (PP)1gamma at mitochondria, thereby inhibiting the activity of the bound enzyme. Growth factor stimulation induces disassembly of URI/PP1gamma complexes through S6K1-mediated phosphorylation of URI at serine 371. This activates a PP1gamma-dependent negative feedback program that decreases S6K1 activity and BAD phosphorylation, thereby altering the threshold for apoptosis. These findings establish URI and PP1gamma as integral components of an S6K1-regulated mitochondrial pathway dedicated, in part, to oppose sustained S6K1 survival signaling and to ensure that the mitochondrial threshold for apoptosis is set in accord with nutrient and growth factor availability.

BAD: undertaker by night, candyman by day

“…S136 is a preferred substrate for AKT and p70S6 kinases in the PI3K signaling pathway (Datta et al, 1997;del Peso et al, 1997;Blume-Jensen et al, 1998;Eves et al, 1998;Harada et al, 2001). S112 and S155 harbor bona fide protein kinase A (PKA) consensus sites that can also be recognized by p90 ribosomal S6 kinase (p90RSK), a kinase activated by the MAPK pathway that shares multiple common substrates with PKA (Datta et al, 2000;Tan et al, 2000;Houslay, 2006).…”

“…Similar to BAD kinases, BAD phosphatases are context-and cell-type-specific. Several BAD phosphatases have been characterized within specific death paradigms, including calcineurin (Wang et al, 1999), PP2A (Chiang et al, , 2003 and PP1 (Ayllon et al, 2000;Salomoni et al, 2000;Danial et al, 2003;Djouder et al, 2007). These phosphatases may preferentially target selective serine sites in BAD.…”

“…Although S136 is the apical serine the phosphorylation of which is needed for neutralizing BAD's apoptotic function, pS112 dephosphorylation may be the initial dephosphorylation event required for promoting the apoptotic activity of BAD. It is also possible that, in addition to directly targeting specific serine sites, BAD phosphatases may inactivate the survival kinases (Andjelkovic et al, 1996;Djouder et al, 2007). Mitochondria-tethered signaling complexes relay survival signals to BAD phosphorylation Among the kinases in charge of BAD phosphorylation, PKA and p70S6K were biochemically purified as mitochondrial-tethered activities that phosphorylate BAD (Harada et al, 1999(Harada et al, , 2001.…”

“…Whether the phosphatase within these mitochondrial signaling complexes directly dephosphorylates BAD or indirectly attenuates the kinase awaits further studies. Recently, a distinct PP1-containing complex was implicated in the attenuation of another BAD survival kinase, p70S6K (Djouder et al, 2007). PP1g was shown to reside in a mitochondrial signaling complex together with Unconventional prefoldin RPB5 Interactor (URI), a member of the prefoldin family of chaperones that normally keeps PP1g in a dormant state (Figure 4).…”

New insights into the biogenesis of nuclear RNA polymerases?This paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process.

“…Moreover, URI/RMP phosphorylation leads to the release of the phosphatase PP1γ from its tethered state at the surface of mitochondria and subsequently acts in a negative feedback loop to alleviate the effect of S6K1-mediated phosphorylation of the apoptotic protein BAD (Djouder et al 2007). This ability of URI/RMP to bind PP1α-type phosphatases, of which human PP1γ is a member, is conserved even in Drosophila (Kirchner et al 2008).…”