Cell Science at a Glance 5675 Protein kinase B/Akt at progression. Conserved from primitive This architecture is conserved among metazoans to humans, PKB belongs to the species from fly, worm, mouse to man. a glance AGC subfamily of the protein kinase Both the central kinase domain and the Elisabeth Fayard*, Lionel A. superfamily, which consists of 518 HM are highly conserved among AGC members in humans (Manning et al., members, such as protein kinase C, Tintignac*, Anne Baudry and 2002). PKB and v-AKT were first cloned ribosomal p90 S6 kinase, serum- and Brian A. Hemmings‡ in 1991 by three independent groups glucocorticoid-induced kinase, and p70 Friedrich Miescher Institute for Biomedical (Brazil and Hemmings, 2001), more than S6 kinase, mediators of responses Research, Maulbeerstrasse 66, 4058 Basel, Switzerland a decade after the original identification triggered by insulin and/or other growth *These authors contributed equally to this work of transforming murine leukemia virus factors. Interestingly, the PH domain, ‡Author for correspondence Akt8 (Staal et al., 1977). The PKB essential for binding to lipids such as (e-mail: [email protected]) subfamily comprises three mammalian PtdIns(3,4,5)P3, is absent in the S. ␣ ␤ ␥ Journal of Cell Science 118, 5675-5678 isoforms, PKB , PKB and PKB cerevisiae orthologue (Sch9). Published by The Company of Biologists 2005 (Akt1, Akt2 and Akt3, respectively), doi:10.1242/jcs.02724 which are products of distinct genes and share a conserved structure that includes PKB regulation Among the signalling proteins that three functional domains: an N-terminal The key role of PKB in signalling respond to a large variety of signals, pleckstrin homology (PH) domain, a became obvious when it was shown protein kinase B (PKB, also known as central kinase domain, and a C-terminal to be a downstream component Akt) appears to be a central player in regulatory domain containing the in phosphoinositide (PI) 3-kinase regulation of metabolism, cell survival, hydrophobic motif (HM) phosphorylation signalling, which is activated upon (1) motility, transcription and cell-cycle site [FxxF(S/T)Y] (Hanada et al., 2004). autophosphorylation of receptor tyrosine Protein Kinase B/Akt at a Glance Elisabeth Fayard, Lionel A. Tintignac, Anne Baudry and Brian A. Hemmings PKB-knockout mice PKB signalling in cancer Targeted isoform Survival Phenotype Cancer type Type of alteration PKBα/AKT1 GF PKBα/AKT2 Integrin GFR PKBα/AKT3 PTEN PKBα/AKT1 PKBβ/AKT2 SHIP P P wortmannin LY294002 PtdIns GPCR (4,5)P SHC 2 P P P P P P85 P110 (3 tdIn Grb2 ,4 s PtdIns P ,5) P CTMP P P P P (3,4,5)P Plasma membrane 3 PtdIns 3 Journal of Cell Science P ? (3,4,5)P TRB3 Paxillin SOS 3 P P P Keratin 10 P P Activity Identified S473 Activity FAK Ras T308 T308 x1000 upstream kinases PI3K x100 P P LY492002 PDK1 DNA-PK LY492002 Independent activation TORC2 i.e. stimulus-specific, LY492002 ATM cell-type-specific S473 S473 ␤ P PKC II Hsp90, Hsp27, MAPKAP-K2 PKB ternary structure Grb10, Ft1 PKB family phylogeny Angiogenesis Nematode 2 Nematode 1 eNOS Human 1 Caw Rat 1 Mouse 1 Chicken tion P Clawed frog a WNK1 loc Zebrafish s Human 2 n Rat 2 a P Tr IRS1 Mouse 2 Mouse 3 Rat 3 Human 3 6-PF2K Fruitfly Mosquito Metabolism PP2A n Inactivatio TSC2 PKB family structure Inactive e PH PHLPP as Thr-308Ser-473 P P GSK3 Kin PKB␣/Akt1 PH Kinase domain HM 480 HM Thr-309 Ser-474 IKK? BRF1 jcs.biologists.org P P FOXO1 ␤ Kinase domain T PKB /Akt2 PH HM Myt1 ra 481 n s Nuclear export Thr-305 Ser-472 PKB lo Mdm2 c P P a tio PLK1 PKB␥/Akt3 PH Kinase domain HM n ␬ ␣ ␬ 479 Raf I B NF- B Cytoplasm P P p21 p27* v-Akt Viral gag PH Kinase domain HM 755 Caspase 9* Cytoplasmic retention Nucleus Bad P Bax P NF-␬B Cell growth Symbol p53 Protein synthesis p27* Natural inhibitor of Inhibition of the target upon Transcription Mdm2 the PI3/PKB pathway PKB-mediated phosphorylation Mitochondria p21 Natural activator Activation of the target upon Cell survival FOXO4 FOXO3a of PKB PKB-mediated phosphorylation Degradation Phosphosite identified only in Chemical inhibitor the human protein Abbreviations: BAD, Bcl2 antagonist of cell death; BRF1, butyrate response factor 1; CTMP, C-terminal modulator protein; DNA PK, DNA-dependent protein kinase; eNOS, endothelial nitric oxide synthase; FAK, focal adhesion kinase; FOXO1, -3a, -4, Forkhead Box O1, -3a, -4 (FOXO1/FKHR, FOXO3a/FKHRL1 and FOXO4/AFX); Ft1, fused toes protein 1; GPCR, G-protein-coupled receptor; Grb, growth factor receptor bound protein; GSK3, glycogen synthase kinase 3; Hsp27, heat shock protein 27 kDa; Hsp90, heat shock protein 90 kDa; IKK, inhibitor of κ light chain enhancer in B cells; MAPKAP-K2, mitogen activated protein kinase activated protein kinase 2; Mdm2, mouse double minute 2; TORC2, TOR ␬ ␬ (target of rapamycin) complex 2; Myt1, membrane-associated and tyrosine/threonine-specific 1; NF- B, nuclear factor B; PDK1, 3-phosphoinositide-dependent protein kinase; PtdIns(4,5)P2, phosphatidylinositol (4,5)-bisphosphate; PtdIns(3,4,5)P3, phosphatidylinositol (3,4,5)-trisphosphate; PHLPP, PH domain and leucine-rich-repeat protein phosphatase; 6-PF2K, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase; PKC II, protein kinase C isoform beta 2; Plk1, Polo-like kinase1; PP2A, protein phosphatase 2 subunit A; Ras, rat sarcoma oncogene; SHC, Src homology 2 domain-containing transforming protein 1; SHIP, SH2-domain-containing inositol polyphosphate 5-phosphatase; SOS, Son of Sevenless. ©Journal of Cell Science 2005 (118, pp. 5675-5678) (See poster insert) 5676 Journal of Cell Science 118 (24) kinases induced by ligands (such as depend on the cellular and physiological isoforms still needs to be addressed, but insulin or other growth factors), (2) context. Phosphorylation of Ser473 is recent work on the three knockout mice stimulation of G-protein-coupled the key step in the activation of PKB suggests isoform-specific functions, as receptors, or (3) activation of integrin because it stabilizes the active well as some functional redundancy signalling (Foster et al., 2003; Wymann et conformation state (Yang et al., 2002). (Yang et al., 2004). al., 2003). PI 3-kinase is the key enzyme Once activated at the plasma membrane, in the generation of the second messenger phosphorylated PKB can translocate to Genetic manipulation of Pkb genes in PtdIns(3,4,5)P3 from PtdIns(4,5)P2. This the cytosol or the nucleus (Andjelkovic mice, either by transgenic or knockout allows the translocation of PKB from the et al., 1999). approaches, has brought novel insights cytoplasm to the plasma membrane, into PKB signalling (Brazil et al., 2004; which involves its PH domain. Interestingly, a number of PKB-binding Yang et al., 2004 and references therein). Interestingly, artificial targeting of PKB proteins have been shown to modulate These studies identified a wide range of to the plasma membrane, using a PKB activity further in either a positive in vivo functions of PKB, revealing myristoylated/palmitoylated sequence or negative manner. This suggests tissue-specific requirements for the motif fused to the N-terminus of PKB, transient regulation of the kinase by different PKB isoforms in metabolism induces constitutive activation of the adaptator proteins (Brazil et al., 2002; and embryonic development. Various kinase. Once recruited to the plasma Song et al., 2005). Moreover, several transgenic mice expressing membrane, PKB is activated by a multi- phosphatases negatively regulate PKB constitutively active PKB under the step process that requires phosphorylation activity. The tumor suppressor control of tissue-specific promoters have of both Thr308 in the activation loop of phosphatase and tensin homology been generated. From the analysis of the kinase domain and Ser473 within the deleted on chromosome ten (PTEN) these, it appears that PKB plays a major HM of the regulatory domain. (Stambolic et al., 1998) and role in cell growth: overexpression of Phosphorylation and subsequent the SH2-domain-containing inositol active PKB leads to cardiac or skeletal activation are prevented by the PI 3- polyphosphate 5-phosphatase (SHIP) muscle hypertrophy and an increase in kinase inhibitors wortmannin and (Huber et al., 1999) inhibit PKB activity ␤-cell mass and size, as well as LY294002, both of which are key indirectly by converting PtdIns(3,4,5)P3 potentially promoting the progression of diagnostic reagents for delineating to PtdIns(4,5)P2 and PtdIns(3,4)P2. tumours such as T cell lymphoma or signalling events associated with this Protein phosphatase 2A (PP2A) and PH prostate intraepithelial neoplasia (Lai et pathway. domain leucine-rich repeat protein al., 2004; Yang et al., 2004). phosphatase (PHLPP␣) do so directly by Interestingly, recent results from The serine/threonine kinase phospho- dephosphorylating Ser473 and/or transgenic mice expressing a kinase- inositide-dependent kinase 1 (PDK1), Thr308 on PKB (Andjelkovic et al., dead mutant of PKB␣ in ␤ cells suggest once recruited to the plasma membrane 1996; Gao et al., 2005). Although PKB that it is also involved in the regulation by PtdIns(3,4,5)P3 through its PH is also proposed to be activated in a PI of the insulin secretory pathway (Bernal- Journal of Cell Science domain, is the kinase responsible for the 3-kinase-independent manner Mizrachi et al., 2004). Expression of phosphorylation of Thr308 (Alessi et al., (Vanhaesebroeck and Alessi, 2000), the activated PKB␣ in the mammary gland 1997; Stephens et al., 1998). However, physiological significance of these results in delayed involution and the the identity of the Ser473 kinase findings requires further study. induction of lipid synthesis, leading to an (S473K, also called PDK2) is still increase in milk fat content and lactation controversial.