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PKB/Akt

The PDK1–PKB/Akt axis represents one of the most actively researched cell signaling pathways. This protein kinase cascade is known to play a central role in mediating the actions of a range of stimuli including insulin, growth factors, integrins and GPCRs in addition to being involved in the regulation of cell survival, cellular metabolism (including insulin-stimulated glucose transport and glycogen synthesis), gene expression, cell cycle entry and protein synthesis.

All the kinases associated with this pathway lie in the protein serine/threonine kinase family and form a single highly branching protein kinase cascade (hence their grouping together). Several of these kinases contain pleckstrin homology (PH) domains that bind specific phosphoinositide lipids (e.g. phosphoinositide-3,4,5-trisphosphate; PIP3) which are generated in the plasma membrane in response to agonist activity. As a result, the kinases are activated in a phosphoinositide 3-OH-kinase (PI3-kinase)- dependent manner.

3-Phosphoinositide-dependent protein kinase-1 (PDK1) stands at the head of this important signaling pathway. Whether extracellular stimuli directly activate PDK1 (perhaps via the generation of plasma membrane-localized PIP3), or whether they simply induce the translocation of PDK1 to its substrate proteins within the plasma membrane, is not known. PDK1 activates a number of AGC-family protein kinases (named after their homology to protein kinases A, G and C) by phosphorylation, including protein kinase B (PKB or Akt) via phosphorylation of the T-loop residue Thr 308. The full activation of PKB/Akt also involves the binding of PIP3 to the PH domain of PKB/Akt and the phosphorylation of an additional residue, Ser 473, by an as yet unidentified kinase called “PDK2” (proposed to be DNA-dependent protein kinase: DNA-PK). There is a great deal of functional overlap between PKB/Akt isoforms; all phosphorylate the same RXRXXS/T motif and all are capable of transforming a cell when rendered constitutively active by the introduction of a myristoylation signal sequence.

Thr 308 and Ser 473 lie within regions of PKB/Akt that are conserved throughout the AGC family kinases. Hence, PDK1 also phosphorylates and activates several other AGC-family kinases, including the serum and glucocorticoid-induced kinases (SGK), atypical forms of protein kinase C (e.g. PKCζ and PKCι/λ), p70S6-kinase and p90RSK. PDK1 is therefore a central controller of multiple cell signaling pathways.

Once phosphorylated and activated, PKB/Akt phosphorylates and inhibits glycogen synthase kinase 3 (GSK3) leading to a decreased phosphorylation and activation of glycogen synthase. PKB/Akt also phosphorylates the mammalian target of rapamycin (mTOR, also known as FRAP and RAFT) although the role of this action is not yet known. GSK3 continues to grow in importance as it also plays a role in the regulation of β-catenin stability and thus gene expression.

mTOR, which can be phosphorylated by PKB/Akt, is unusual in that it possesses both serine/threonine protein kinase as well as lipid kinase activities. It is a large complex molecule that is a receptor for the immunosuppressant, rapamycin. mTOR, along with PDK1, then plays an as yet ill-defined role in the activation of p70S6K which is important in the control of protein synthesis, development and growth control. Thus, at least in part, the immunosuppressive activity of rapamycin is due to its actions on mTOR.

Other than rapamycin, there are few if any highly specific pharmacological inhibitors of this collection of protein kinases. This has made understanding the role of these protein kinases in mediating the effects of extracellular stimuli very difficult to ascertain. Furthermore, while in some cases kinase-dead derivatives of these kinases have been reported to act as dominant-negatives, this has often been highly controversial, largely because of their complex domain structures and abilities to interact with other proteins and signaling lipids. However, given the central importance of these protein kinases in numerous disease states (e.g. cancer and diabetes), the identification of specific inhibitors remains a very important goal.

 

The Table below contains accepted modulators and additional information. For a list of additional products, see the "Similar Products" section below.

 

Family Members PDK1 PKB/Akt SGK
Other Names 3-Phosphoinositide protein kinase-1 RAC-protein kinase Serum and glucacorticoid-induced kinase
CISK (SGK3)
Molecular Weight
(kDa)
63 kDa 56 kDa (All isoforms) SGK1: 49 kDa
SGK2α: 48 kDa
SGK2β: 41 kDa
SGK3 57: kDa
Structural
Data
556 aa
Monomer
PKBα: 480 aa
PKBβ: 481 aa
PKBγ: 479 aa
All monomers
SGK1: 431 aa
SGK2α: 367 aa
SGK2β: 427 aa
SGK3: 496 aa
All monomers
Isoforms Not Known PKBα/Akt1
PKBβ/Akt2
PKBγ/Akt3
SGK1
SGK2α/2β (splice variants)
SGK3
Species All eukaryotes All metazoans
(kinases orthologous to PKB and SGK exist in
fungi, plasmodium, dictystelium)
All metazoans
(kinases orthologous to PKB and SGK exist in
fungi, plasmodium, dictystelium)
Domain
Organization
1 PH domain binds PtdIns(3,4,5)P3 1 PH domain binds
PtdIns(3,4,5)P3
PtdIns(3,4)P2
Protein kinase domain
Phosphorylation
Sites
Ser241 (autophosphorylation) PKBα sites, but conserved:
Thr308
Ser473
Thr450 (constitutive)
Tyr315
Tyr326
SGK1 sites, but conserved:
Thr256
Ser442
Ser78
Tissue
Distribution
Ubiquitous Ubiquitous Ubiquitous
Subcellular
Localization
Cytosolic
Membranes
Cytosolic
Membranes
Cytosolic
Membranes (SGK3)
Binding Partners/
Associated Proteins
Not Known CTMP
APPL
Periplakin
POSH
HSP90
PKCθ
Brk
PKCζ
IMPDH
Ft1
TCL1
Not Known
Upstream
Activators
Complex-see Overview Phosphorylation by PDK1 and "PDK2"
and binding of PIP3
Phosphorylation by PDK1
(SGK1 expression induced by glucocorticoids)
Downstream
Activation
PKA (P5511, C8482, P2645)
PKG
PKB/Akts
MSKs
atypical PKCs
p70S6Ks
p90RSKs (all are protein kinases)
GSK3
mTOR
Raf
IKK
BAD
eNOS
hCaspase-9 (C1099)
hTERT
BRCA1
IRS-1
PFK2
FKHR
PRAS40
TSC2
AS160
PIKfyve
hdm-2
WNK1
FKHR
NDRG1
NDRG2
Nedd4-2
B-Raf
Activators Not Known Not Known Not Known
Inhibitors UCN-01 (U6508)
SH5
API-2 (T3830)
AKT-I-1
Akt-I-1.2
Not Known
Selective
Activators
Not Known Not Known Not Known
Physiological
Function
Cell proliferation
Growth
Apoptosis
Metabolism
Cell proliferation
Growth
Apoptosis
Metabolism
Regulation of Na+ and Cl- channels
Disease
Relevance
Cancer
Inflammation
Others
Cancer
Inflammation
Others
Hypertension

 

 

Family Members GSK3 mTOR p70S6K (S6K)
Other Names Glycogen synthase kinase 3 (G1663) FRAP
RAFT
Mammalian target of rapamycin
S6K1
Molecular Weight
(kDa)
GSK3α: 51 kDa
GSK3β: 48 kDa
289 kDa α1: 59 kDa
α2: 56 kDa
β1: 55 kDa
β2: 54 kDa
Structural
Data
483 aa
β: 420 aa
All monomers
2549 aa
Monomer
α1: 525 aa
α2: 502 aa
β1: 495 aa
β2: 482 aa
All monomers
Isoforms GSK3α
GSK3β
Not Known p85 S6Kα1 = α1
p70 S6Kα2 = α2
(splice variants of the S6K1 gene)
p60 S6Kβ1 = β1
p54 S6Kβ2 = β2
(splice variants of the S6K2 gene)
Species All eukaryotes All eukaryotes All metazoans
Domain
Organization
Not Known 16 HEAT domains
1 PI3-kinase homology domain
1 FKBP/rapamycin binding domain
1 autoinhibitory domain
1 nuclear localization signal sequence on α1, β1 and β2
Phosphorylation
Sites
GSK3α, but conserved in GSK3β:
Ser21 (PKB and others)
Tyr296 (constitutive)
Ser2448
Ser2481 (autophosphorylation)
Sites α2, but conserved:
Thr229
Ser371
Ser389
Ser411
Ser418
Thr424
In α2, conserved in α1 only:
Thr421
Tissue
Distribution
Ubiquitous Ubiquitous Ubiquitous
Subcellular
Localization
Cytosolic
Nuclear
Cytosolic Cytosolic (α2)
Nuclear (α1, β1 and β2)
Binding Partners/
Associated Proteins
Frat-1/2/3
Presenilin
Axin
Axil
DLP
14-3-3
FKBP12
Raptor
Rictor
Rheb
LST8
PP2A
Neurabin
Upstream
Activators
Tyrosine phosphorylation (constitutive)
Phosphorylation by PKB/Akt
Complex, but may include phosphorylation
by PKB and regulation by TSC2/Rheb
Phosphorylation by PDK1, mTOR and atypical PKCs
Downstream
Activation
Glycogen synthase
ATP-citrate lyase
eIF-2B
c-Jun
Myc
Myb
PKA (P5511, C8482, P2645)
CREB
IkB
PP1 (P7937)
tau
NDRG1
4E-BP1
p70S6K
Ribosomal S6 subunit
SKAR
Activators Not Known Not Known Not Known
Inhibitors SB-216763 (S3442)
SB-415286 (S3567)
Lithium Chloride (213233)
CT99021
AR-A014418 (A3230)
Rapamycin (R0395) H89 (B1427)
Selective
Activators
Not Known Not Known Not Known
Physiological
Function
Glycogen metabolism
Gene expression
Development
Protein synthesis
Cell growth
Protein synthesis
Cell growth
Disease
Relevance
Diabetes
Metabolism
Cancer
Immunosuppression
Cancer

 

Footnotes

a) Inhibitors in parentheses are non-selective or yet unproven to be highly specific.

 

Abbreviations

Brk: Breast tumor-related kinase
CTMP: C-terminal modulatory protein
DLP: Dynamin-like protein
DNA-PK: DNA-dependent protein kinase
FKBP12: FK502 binding protein
FRAP: FKBP12-rapamycin-associated protein
GSK3: Glycogen synthase kinase 3
H89: N-(2-[p-Bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide
IMPDH: Inosine-5’ monophosphate dehydrogenase
MSK: Mitogen- and stress-activated kinase
MTOR: Mammalian target of rapamycin
PIKfyve: FYVE domain containing phosphatidylinositol 3-phosphate 5-kinase
PKA: Protein kinase A
PKC: Protein Kinase C
PKG: Protein kinase G
POSH: Plenty of SH3 domains
PtdIns(3,4)P2: Phosphatidylinositol 3,4-bisphosphate
PtdIns(3,4,5)P3: Phosphatidylinositol 3,4,5-trisphosphate
RAFT: Rapamycin and FKBP12 target
Ro 31-8220: 2-{1-[3-Amidinothio)propyl]-1H-indol-3-yl}-3-(1-methylindol-3-yl)-maleimide
SB-216763: 3-(2,4-Dichlorophenyl)-4-(1-methyl-1H-indole-3-yl)-1H-pyrrole-2,5-dione
SB-415286: 3-(3-Chloro-4-hydroxyphenylamino)-4-(2-nitrophenyl)-1H-pyrrole-2,5-dione
SGK: Serum and glucocorticoid regulated kinase
TCL: T-cell leukemia
TSC2: Tuberous Sclerosis Complex-2
UCN-1: 7-Hydroxystaurosporine

 

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References