The Journal of biological chemistry
-
Gln3p is a GATA-type transcription factor responsive to different nitrogen nutrients and starvation in yeast Saccharomyces cerevisiae. Recent evidence has linked TOR signaling to Gln3p. Rapamycin causes dephosphorylation and nuclear translocation of Gln3p, thereby activating nitrogen catabolite repressible-sensitive genes. ⋯ The yeast pro-prion protein Ure2p binds to both hyper- and hypo-phosphorylated Gln3p. In contrast to the free Gln3p, the Ure2p-bound Gln3p is signifcantly resistant to dephosphorylation. Taken together, these results reveal a tripartite regulatory mechanism by which the phosphorylation of Gln3p is regulated.
-
G-protein-gated inwardly rectifying K(+) (GIRK) channels are widely expressed in the brain and are activated by at least eight different neurotransmitters. As K(+) channels, they drive the transmembrane potential toward E(K) when open and thus dampen neuronal excitability. There are four mammalian GIRK subunits (GIRK1-4 or Kir 3.1-4), with GIRK1 being the most unique of the four by possessing a long carboxyl-terminal tail. ⋯ The most significant difference between the putative GIRK2/GIRK3 heteromultimeric channel and GIRK1/GIRKx channels at the single channel level was an approximately 5-fold lower sensitivity to activation by Gbetagamma. Complexes containing only GIRK2 and GIRK3 could be immunoprecipitated from transfected cells and could be purified from native brain tissue. These data indicate that functional GIRK channels composed of GIRK2 and GIRK3 subunits exist in brain.
-
Glycogen synthase kinase 3beta (GSK3beta) is a key component in many biological processes including insulin and Wnt signaling. Since the activation of each signaling pathway results in a decrease in GSK3beta activity, we examined the specificity of their downstream effects in the same cell type. Insulin induces an increased activity of glycogen synthase but has no influence on the protein level of beta-catenin. ⋯ Although the decrease in GSK3beta activity is required, GSK3beta may not be the limiting component for Wnt signaling in the cells that we examined. Our results suggest that the axin-conductin complexed GSK3beta may be dedicated to Wnt rather than insulin signaling. Insulin and Wnt pathways regulate GSK3beta through different mechanisms, and therefore lead to distinct downstream events.
-
Intracellular iron homeostasis is regulated, in part, by interactions between iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellular oxidative stress induced by H(2)O(2), nitric oxide, and hypoxia, and hormonal activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated through protein kinase C (PKC), play a central role in the modulation of IRP/IRE interactions since phorbol esters were shown to activate IRP binding (Eisenstein, R. ⋯ Interestingly, ferritin protein levels were regulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP binding and ferritin expression. Remarkably, for TRH and EGF, these effects may be PKC-dependent or -independent determined by the cell type.
-
The Gab family of docking proteins is phosphorylated in response to various growth factors and cytokines and serves to recruit multiple signaling proteins. Gab1 acts downstream from the Met-hepatocyte growth factor receptor, and Gab1 overexpression promotes Met-dependent morphogenesis of epithelial cells. Recruitment of Gab1 to Met or epidermal growth factor (EGF) receptors requires a receptor-binding site for the Grb2 adapter protein and a proline-rich domain in Gab1, defined as the Met-binding domain. ⋯ The PXXXR motif is required but not sufficient for Grb2 binding, whereas an extended motif, PX3RX2KPX7PLD, conserved in Gab proteins as well as the Grb2/Gads-docking protein, Slp-76, efficiently competes binding of Grb2 or Gads adapter proteins. The association of Gab1 with Grb2 is required for Gab1 recruitment to the EGF receptor but not the Met receptor. Hence different mechanisms of Gab1 recruitment may reflect the distinct biological functions for Gab1 downstream from the EGF and Met receptors.