|The Akt isoforms are present at distinct subcellular locations.|
Santi, SA; Lee, H
American journal of physiology. Cell physiology
Akt is involved in the regulation of diverse cellular functions such as cell proliferation, energy metabolism, and apoptosis. Although three Akt isoforms are known, the function of each isoform is poorly understood. To gain a better understanding of each Akt isoform, we examined the subcellular localization and expression of each isoform in transformed and nontransformed cells. Akt1 was localized in the cytoplasm, which is in agreement with the currently accepted model that cytoplasmic Akt is translocated and activated at the inner leaflet of the plasma membrane. Interestingly, HEK-293 and HEK-293T cells contained Akt1 in the nucleus and cytoplasm, respectively, suggesting that SV40 T-antigen plays a crucial role in the cytoplasmic localization and activation of Akt1 in HEK-293T. Akt2 was colocalized with the mitochondria, while Akt3 was localized in both the nucleus and nuclear membrane. The subcellular localization of the Akt isoforms was not substantially altered in response to ionizing radiation or EGF. Furthermore, the ablation of one Akt isoform by small interfering RNA (siRNA) did not alter the subcellular location of the remaining isoforms, suggesting that the major function of one isoform is not compensated for by other isoforms. Together, our data support the notion that Akt2 and Akt3 are regulated at the mitochondrial and nuclear membranes, respectively. The mitochondrial localization of Akt2 raises the possibility that this isoform may be involved in both glucose-based energy metabolism and suppression of apoptosis, two Akt functions previously identified with anti-pan-Akt antibodies.
|Characterization of PDK2 activity against protein kinase B gamma.|
Hodgkinson, Conrad P, et al.
Biochemistry, 41: 10351-9 (2002)
Protein kinase B (PKB), also known as Akt, is a serine/threonine protein kinase controlled by insulin, various growth factors, and phosphatidylinositol 3-kinase. Full activation of the PKB enzyme requires phosphorylation of a threonine in the activation loop and a serine in the C-terminal tail. PDK1 has clearly been shown to phosphorylate the threonine, but the mechanism leading to phosphorylation of the serine, the PDK2 site, is unclear. A yeast two-hybrid screen using full-length human PKBgamma identified protein kinase C (PKC) zeta, an atypical PKC, as an interactor with PKBgamma, an association requiring the pleckstrin homology domain of PKBgamma. Endogenous PKBgamma was shown to associate with endogenous PKCzeta both in cos-1 cells and in 3T3-L1 adipocytes, demonstrating a physiological interaction. Immunoprecipitates of PKCzeta, whether endogenous PKCzeta from insulin-stimulated 3T3-L1 adipocytes or overexpressed PKCzeta from cos-1 cells, phosphorylated S472 (the C-terminal serine phosphorylation site) of PKBgamma, in vitro. In vivo, overexpression of PKCzeta stimulated the phosphorylation of approximately 50% of the PKBgamma molecules, suggesting a physiologically meaningful effect. However, pure PKCzeta protein was incapable of phosphorylating S472 of PKBgamma. Antisense knockout studies and use of a PDK1 inhibitor showed that neither PKB autophosphorylation nor phosphorylation by PDK1 accounted for the S472 phosphorylation in PKCzeta immunoprecipitates. Staurosporine inhibited the PKCzeta activity but not the PDK2 activity in PKCzeta immunoprecipitates. Together these results indicate that an independent PDK2 activity exists that physically associates with PKCzeta and that PKCzeta, by binding PKBgamma, functions to deliver the PDK2 to a required location. PKCzeta thus functions as an adaptor, associating with a staurosporine-insensitive PDK2 enzyme that catalyzes the phosphorylation of S472 of PKBgamma. Because both PKCzeta and PKB have been proposed to be required for mediating a number of crucial insulin responses, formation of an active signaling complex containing PKCzeta, PKB, and PDK2 is an attractive mechanism for ensuring that all the critical sites on targets such as glycogen synthase kinase-3 are phosphorylated.