NH2 terminus of serum and glucocorticoid-regulated kinase 1 binds to phosphoinositides and is essential for isoform-specific physiological functions

Alan C. Pao, James (Jim) McCormick, Hongyan Li, John Siu, Cedric Govaerts, Vivek Bhalla, Rama Soundararajan, David Pearce

Research output: Contribution to journalArticle

28 Citations (Scopus)

Abstract

Serum and glucocorticoid regulated kinase 1 (SGK1) has been identified as a key regulatory protein that controls a diverse set of cellular processes including sodium (Na+) homeostasis, osmoregulation, cell survival, and cell proliferation. Two other SGK isoforms, SGK2 and SGK3, have been identified, which differ most markedly from SGK1 in their NH2- terminal domains. We found that SGK1 and SGK3 are potent stimulators of epithelial Na+ channel (ENaC)-dependent Na+ transport, while SGK2, which has a short NH2 terminus, is a weak stimulator of ENaC. Further characterization of the role of the SGK1 NH2 terminus revealed that its deletion does not affect in vitro kinase activity but profoundly limits the ability of SGK1 either to stimulate ENaC-dependent Na + transport or inhibit Forkhead-dependent gene transcription. The NH2 terminus of SGK1, which shares sequence homology with the phosphoinositide 3-phosphate [PI(3)P] binding domain of SGK3, binds phosphoinositides in protein lipid overlay assays, interacting specifically with PI(3)P, PI(4)P, and PI(5)P, but not with PI(3,4,5)P3. Moreover, a point mutation that reduces phosphoinositide binding to the NH2 terminus also reduces SGK1 effects on Na+ transport and Forkhead activity. These data suggest that the NH2 terminus, although not required for PI 3-kinase-dependent modulation of SGK1 catalytic activity, is required for multiple SGK1 functions, including stimulation of ENaC and inhibition of the proapoptotic Forkhead transcription factor. Together, these observations support the idea that the NH2-terminal domain acts downstream of PI 3-kinase-dependent activation to target the kinase to specific cellular compartments and/or substrates, possibly through its interactions with a subset of phosphoinositides.

Original languageEnglish (US)
JournalAmerican Journal of Physiology - Renal Physiology
Volume292
Issue number6
DOIs
StatePublished - Jun 2007

Fingerprint

Phosphatidylinositols
Protein Isoforms
Epithelial Sodium Channels
Phosphatidylinositol 3-Kinases
Phosphotransferases
Forkhead Transcription Factors
Osmoregulation
serum-glucocorticoid regulated kinase
Sequence Homology
Point Mutation
Cell Survival
Proteins
Homeostasis
Sodium
Cell Proliferation
Lipids

Keywords

  • Epithelial sodium channel
  • Forkhead
  • Kinase activity

ASJC Scopus subject areas

  • Physiology

Cite this

NH2 terminus of serum and glucocorticoid-regulated kinase 1 binds to phosphoinositides and is essential for isoform-specific physiological functions. / Pao, Alan C.; McCormick, James (Jim); Li, Hongyan; Siu, John; Govaerts, Cedric; Bhalla, Vivek; Soundararajan, Rama; Pearce, David.

In: American Journal of Physiology - Renal Physiology, Vol. 292, No. 6, 06.2007.

Research output: Contribution to journalArticle

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AU - Siu, John

AU - Govaerts, Cedric

AU - Bhalla, Vivek

AU - Soundararajan, Rama

AU - Pearce, David

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AB - Serum and glucocorticoid regulated kinase 1 (SGK1) has been identified as a key regulatory protein that controls a diverse set of cellular processes including sodium (Na+) homeostasis, osmoregulation, cell survival, and cell proliferation. Two other SGK isoforms, SGK2 and SGK3, have been identified, which differ most markedly from SGK1 in their NH2- terminal domains. We found that SGK1 and SGK3 are potent stimulators of epithelial Na+ channel (ENaC)-dependent Na+ transport, while SGK2, which has a short NH2 terminus, is a weak stimulator of ENaC. Further characterization of the role of the SGK1 NH2 terminus revealed that its deletion does not affect in vitro kinase activity but profoundly limits the ability of SGK1 either to stimulate ENaC-dependent Na + transport or inhibit Forkhead-dependent gene transcription. The NH2 terminus of SGK1, which shares sequence homology with the phosphoinositide 3-phosphate [PI(3)P] binding domain of SGK3, binds phosphoinositides in protein lipid overlay assays, interacting specifically with PI(3)P, PI(4)P, and PI(5)P, but not with PI(3,4,5)P3. Moreover, a point mutation that reduces phosphoinositide binding to the NH2 terminus also reduces SGK1 effects on Na+ transport and Forkhead activity. These data suggest that the NH2 terminus, although not required for PI 3-kinase-dependent modulation of SGK1 catalytic activity, is required for multiple SGK1 functions, including stimulation of ENaC and inhibition of the proapoptotic Forkhead transcription factor. Together, these observations support the idea that the NH2-terminal domain acts downstream of PI 3-kinase-dependent activation to target the kinase to specific cellular compartments and/or substrates, possibly through its interactions with a subset of phosphoinositides.

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