14-3-3-mediated Signaling

The 14-3-3 proteins are a family of conserved adaptor and scaffolding proteins expressed in all eukaryotic cells. There are seven known mammalian 14-3-3 isoforms, (β, ε, γ, η, σ, τ and ζ) named after their elution profile on reversed phase high-performance liquid chromatography. The species initially designated α and Delta are actually the phosphorylated forms of β and ζ. 14-3-3 proteins function as dimers with each monomer able to bind a target. The primary function of the 14-3-3 proteins is to bind phosphoserine/threonine-containing motifs in a sequence-specific manner, e.g., RSxpSxP, mode-1 and RxxxpSxP, mode-2, in a manner analogous to Src-homology 2 and phospho-tyrosine-binding domains that bind phosphotyrosine-containing motifs. 14-3-3 actions can be divided into four categories: inhibition, activation, structural stabilization, or translocation. 14-3-3-associated proteins include those involved in cell cycle control, such as CDC25, Wee1, p53, CDC2, and CDK2; cellular signaling and stress responses, such as Raf1, IGFIR, IRS1, PI3K, PLC, PKC, Cbl, oncogene products BCR, polyomavirus MT, MEKK1 and MEKK4, MLK2, JNK, BAD, and ASK1; transcriptional regulation, such as FKHRL1, DAF, TAZ and histone deacetylase; and cytoskeletal proteins such as keratin K18 and vimentin. The kinase activity of ASK1 is stimulated by TNF-α via members of the TRAF family, TRAF2 and by Fas ligation via the Daxx protein. ASK1 acts as a downstream target of TRAF2 in the JNKpathway leading to transcriptional activation of heterodimer c-Jun and c-Fos.14-3-3 maintains Raf1 in an inactive state in the absence of activation signals but promotes Raf1 activation and stabilizes its active conformation when such signals are received. On activation by GRB2, the small GTP binding protein Ras interacts directly with Raf1 and recruits Raf1 to the plasma membrane. Raf1 then phosphorylates the kinase MEK, leading to stimulation of the ERK signaling, which translocate to the nucleus and phosphorylate the transcription factor Elk1 involved in cell division, or they phosphorylate the RSK. Signaling via cell surface receptors initiates the mitogenic response of cells to many growth factors and is a frequent target for events involved in the process of tumorigenesis. RTK activation via growth factor stimulation or oncogenic mutation initiates several phosphorylation cascades, often involving the production of second messengers such as PIP2, DAG, etc. Activation of Akt promotes cell survival and growth via phosphorylation of a number of substrates including PRAS, GSK-3β, several mammalian homologs of the Caenorhabditis elegans DAF transcription factor, the anti-apoptotic protein BAD, YAP, phosphodiesterase 3B, a Rab GTPase-activating protein, ATP-citrate lyase, and most recently, Tuberous Sclerosis Complex. GSK3-β is a regulatory enzyme that phosphorylates several substrates including τ in a complex containing tubulin and 14-3-3 isoforms in normal brain. The phosphorylation of substrate by Akt result in the subsequent binding of the substrate to 14-3-3, which induce a change in the subcellular localization of a protein. Binding to 14-3-3 also retains phosphorylated YAP in the cytoplasm, resulting in its displacement from the nucleus where it functions as a co-activator of p73-mediated apoptosis. Signaling via Akt also results in activation of p70S6K, which phosphorylates ribosomal protein S6K to promote protein synthesis and cell growth.In summary, 14-3-3 act as allosteric cofactors to modulate the catalytic activity or conformational state of its targets.