The 14-3-3 proteins.

What are they? The 14-3-3 proteins make up a ubiquitous family that is expressed in all eukaryotic cells. They are small, about 28–30 kDa, acidic and relatively abundant.How did they get their peculiar name? In 1968, Blake Moore and colleagues were trying to identify proteins unique to the brain, using starch electrophoresis. The name refers to both the fraction number and its position on the gel. But it turned out that 14-3-3 was not brain-specific.So what’s so interesting about them? In 1996 14-3-3 was found to bind to a variety of oncoproteins including Raf–1, polyoma middle T, and Bcr–Abl. Since then the 14-3-3 proteins have been shown to bind to a wide variety of proteins involved in signal transduction, cell cycle and apoptosis. These proteins include Cdc25, NFAT, Bad, Cbl, A20, PI 3-kinase, IRS-1, MEKK, p130Cas, glucocorticoid receptor and the forkhead transcription factor.So is there anything they don’t bind to? The list of proteins to which they bind grows every day. But we now know that 14-3-3 proteins bind to a specific serine phosphorylated sequence. Most of the known 14-3-3-binding proteins contain this motif.All this binding — they must have help? Many serine/threonine kinases are capable of generating 14-3-3 binding sites, including protein kinase A and protein kinase C. Recently, attention has focused on serine kinases regulated by cell survival signals or DNA damage, such as Chk1 and Akt.So what do they do? Almost everything. We know they are involved in signal transduction, cell cycle control, vesicular transport and apoptosis. They also have something to do with responses to growth factors, DNA damage and oncogenesis. For some enzymes, 14-3-3 proteins function as an essential co-factor. For others, they can function as an inhibitor.How do they work? Much recent work suggests that 14-3-3 binding is involved in intracellular targetting. For nuclear proteins such as Cdc25 and NFAT, 14-3-3 binding keeps them in the cytoplasm and out of the nucleus. For apoptotic proteins such as BAD and A20, it keeps them away from membranes. 14-3-3 may also have chaperone-like functions for some proteins.What about this family business? In yeast, Caenorhabditis elegans and Drosophila, there are two 14-3-3 isoforms; in mammalian cells there are seven. Each seems to bind to the same serine-phosphorylated sequence. But there are tissue-specific differences in 14-3-3 expression and one isoform, σ, is regulated by p53 and can block the cell cycle. Understanding the particular function of each isoform is one of the immediate challenges.Do they have commercial potential? Drug companies have shied away from 14-3-3 proteins because of their ubiquitous expression and their promiscuous binding habits. But important roles for the protein family in cell cycle control and apoptosis suggest that a drug that at least partially blocks 14-3-3 binding could potentiate chemotherapeutic agents that induce apoptosis (Figure 1Figure 1).Figure 1Crystal structure of the 14-3-3 protein bound to a serine-phosphorylated peptide. The structure shows that 14-3-3 is dimeric, and capable of binding two ligands simulatneously. The phosphorylated serine interacts with a cluster of basic residues near the bend in the groove. Reproduced with permission from Yaffe MB, et al., Cell 1997 91: 961–971View Large Image | View Hi-Res Image | Download PowerPoint SlideWhere can I find out more?Aitken A: 14-3-3 proteins on the MAP.Trends Biochem Sci 1995, 20:95–97.Fu H, Subramanian RR, Masters SC: 14-3-3 proteins: structure, function, and regulation.Annu Rev Pharm 2000, in press.Hermeking H, Lengauer C, Polyak K, He TC, Zhang L, Thiagalingam S, Kinzler KW, Vogelstein B: 14-3-3 sigma is a p53-regulated inhibitor of G2/M progression. Mol Cell 1997, 1:3–11.Muslin AJ, Tanner JW, Allen PM, Shaw AS: Interaction of 14-3-3 with signaling proteins is mediated by the recognition of phosphoserine.Cell 1996, 84:889–897.Peng CY, Graves PR, Thoma RS, Wu Z, Shaw AS, Piwnica-Worms H: Mitotic and G2 checkpoint control: regulation of 14-3-3 protein binding by phosphorylation of Cdc25C on serine-216. Science 1997, 277:1501–1505.Zha J, Harada H, Yang E, Jockel J, Korsmeyer SJ: Serine phosphorylation of death agonist BAD in response to survival factor results in binding to 14-3-3 not BCL=X(L). Cell 1996, 87:619–628