Multiple members of the 14-3-3 protein family have been found in all eukaryotes so far investigated, yet they are apparently absent from prokaryotes. The major native forms of 14-3-3s are homo- and hetero-dimers, the biological functions of which are to interact physically with specific client proteins and thereby effect a change in the client. As a result, 14-3-3s are involved in a vast array of processes such as the response to stress, cell-cycle control, and apoptosis, serving as adapters, activators, and repressors. There are currently 133 full-length sequences available in GenBank for this highly conserved protein family. A phylogenetic tree based on the conserved middle core region of the protein sequences shows that, in plants, the 14-3-3 family can be divided into two clearly defined groups. The core region encodes an amphipathic groove that binds the multitude of client proteins that have conserved 14-3-3-recognition sequences. The amino and carboxyl termini of 14-3-3 proteins are much more divergent than the core region and may interact with isoform-specific client proteins and/or confer specialized subcellular and tissue localization.
Protein family review
The 14-3-3 protein family is highly conserved and is represented throughout the eukaryotic branch of life. The proteins were discovered in 1967 during a study of the soluble acidic proteins of the mammalian brain  and were named on the basis of fraction number during DEAE-cellulose chromatography and location after starch gel electrophoresis. For 25 years, 14-3-3s were generally thought to reside exclusively in animal brain tissue and to be involved in the function of neurons. During this early period of research, 14-3-3s were characterized as a heterogeneous family of dimeric proteins with a monomer mass of 25-32 kDa and multiple isoelectric points. One of the first biochemical functions of the 14-3-3s to be identified was the activation of the neurotransmitter pathway enzymes tyrosine hydroxylase and tryptophan hydroxylase, in a reaction requiring calcium and the cAMP-dependent kinase or calmodulin-dependent protein kinase II . Once 14-3-3s were found in Arabidopsis thaliana , maize  and other plants, and in tissues other than the brain, however, perspectives on their presence and roles broadened, and now 14-3-3s have been found in every eukaryote that has been screened for their presence. The vast number of organisms containing 14-3-3s suggests that this family of proteins is involved in many important biological processes .
Gene organization and evolutionary history
Each 14-3-3 protein sequence can be roughly divided into three sections: a divergent amino terminus, the conserved core region and a divergent carboxyl terminus. The high level of conservation in the core region is demonstrated in Figure 1a, which shows a similarity plot derived from an alignment of selected 14-3-3s. A phylogenetic tree generated from the full alignment of the core region is shown in Figure 2 and demonstrates that plant 14-3-3s fall into two groups, an epsilon (ε) group and a non-epsilon group [6,7]. The plant non-epsilon group is very different from plant ε isoforms and animal isoforms. Additional complex groupings and subgroupings are apparent, but the tree must be evaluated carefully because not all species are represented by a complete genome sequence . The presence of the divergent termini and the few amino-acid changes that do occur within the conserved region result in multiple isoforms in most organisms and present the potential for client-specific interactions occurring in distinct cellular locations .
Figure 1. Conservation of 14-3-3 proteins. (a) A graph of percentage similarity derived from (b), which shows a multiple sequence alignment of 40 selected 14-3-3 isoforms. Highly conserved helices in (a) are in red and are indicated by numbers that correspond to helices in the crystal structure of Figure 3. The alignment in (b) was created using Clustal W . Residues in red are 100% conserved across all isoforms; residues in blue are highly conserved. These red and blue colors correspond to the red and blue regions of the crystal structure shown in Figure 3.
Figure 2. Phylogenetic relationships of 14-3-3s. This tree is a rooted cladogram from a neighbor-joining analysis  of the 133 different full-length 14-3-3 isoforms that are currently available in the GenBank database , with Dictyostelium discoideum 14-3-3 as the outgroup. Arabidopsis isoforms are highlighted in green. The separation of plant epsilon and non-epsilon 14-3-3 proteins is clearly visible.
Although 14-3-3s are found throughout the eukaryotes, Arabidopsis is an excellent model system for studying 14-3-3s for two reasons: it is a higher eukaryote with a fully sequenced genome and it has a large family of thirteen 14-3-3 isoforms ; there is also a wealth of knowledge on key biological pathways. In Arabidopsis, the 14-3-3 epsilon group has five members (μ (mu), ε, π (pi), ι (iota), and ο (omicron) and the non-epsilon group has eight members (κ (kappa), λ (lambda), ψ (psi), ν (nu), υ (upsilon), ω (omega), (phi), and χ (chi)). The presence and chromosomal location of all Arabidopsis 14-3-3 isoforms is known (Table 1); there is at least one 14-3-3 on each of the five chromosomes. Similar diversity in chromosomal distribution occurs in other species where multiple 14-3-3s have been found.
Table 1. Genetic, cellular and functional information on Arabidopsis 14-3-3s
Characteristic structural features
The conserved middle core region of the 14-3-3s encodes an amphipathic groove that forms the main functional domain, a cradle for interacting with client proteins. Extensive investigation of the 14-3-3 binding site of the mammalian serine/threonine kinase Raf-1 has produced a consensus sequence for 14-3-3-binding, RSxpSxP (in the single-letter amino-acid code, where x denotes any amino acid and p indicates that the next residue is phosphorylated)  which has been verified through peptide library screening . A common, but not exclusive, requirement of 14-3-3 ligands is the phosphorylation of a serine or threonine residue in the target sequence. The phosphorylated consensus sequence does not, however, fully represent every ligand that 14-3-3s can bind: they are also known to bind other non-phosphorylated sequences such as GHSL , and WLDLE . A common, but not exclusive, requirement of 14-3-3 ligands is the phosphorylation of a serine residue in the target sequence. For those client proteins whose target sequences undergo phosphorylation, the binding of 14-3-3s to the target is the major step of a signal-transduction event. Despite the simplicity of the binding-site requirements, a diverse array of proteins potentially interact with 14-3-3s; some reports suggest that as many as 20% of Arabidopsis proteins are clients for 14-3-3s .
Our knowledge of the three-dimensional structure of 14-3-3s is based on the model derived from X-ray diffraction of the crystals of the ζ and τ mammalian isoforms [15,16]. The high level of conservation of 14-3-3 amino acid sequence in the conserved core allows the general features of this structural model to be applied to all known 14-3-3s. The monomer consists of nine α helices organized in an antiparallel manner, forming an L-shaped structure (Figure 3). The interior of the L-structure is composed of four helices: H3 and H5, which contain many charged and polar amino acids, and H7 and H9, which contain hydrophobic amino acids. These four helices form the concave amphipathic groove that interacts with target peptides. An alignment of all currently known full-length isoforms provides evidence that this groove is over 70% conserved (Figures 1 and 3). Five of the most highly conserved regions correspond to helices H1, H3, H5, H7, and h9 (Figure 1). The conserved amphipathic groove is the site for ligand binding; amino acids Lys49, Arg56, and Arg127 in mammalian 14-3-3 ζ sequences have been demonstrated to interact with the phosphorylated amino acids of ligands (serine/threonine kinases Raf-1 and Bcr) by mutational [17,18,19] and co-crystallization [11,13] experiments; the latter have also shown that both phosphorylated and non-phosphorylated ligands bind in the amphipathic groove . The peptides bound by 14-3-3s adopt an extended conformation, which is thought to reduce steric hindrance between neighboring amino acids of the ligand .
Figure 3. The crystal structure of 14-3-3s. The model shown is derived from the human 14-3-3 ζ isoform (PDB1QJB ) and is shown from (a) the top and (b) one side as visualized by the 3Dmol software found in the Vector NTI 7.0 Suite. Helix numbers are denoted from H1 near the amino terminus to H9 near the carboxyl terminus. Red and blue areas correspond to residues of 100% identity and high conservation, respectively, and correspond to colors on the alignment (Figure 1). Yellow areas correspond to regions of reduced similarity and green areas indicate the nuclear export signal .
The 14-3-3 dimerization interaction occurs between the amino-terminal helix H1 of one monomer and helices H3 and H4 of the opposing monomer; the high conservation of amino-acid sequence along helices H1 and H3 among various isoforms allows 14-3-3s to heterodimerize (Figure 3) [21,22]. Two identical or different client proteins can be bound simultaneously by the dimer. This dual interaction means that possible roles of 14-3-3s include acting as adapters capable of bringing disparate client proteins together or moving or rearranging two different regions of the same protein. An example of the potential role of 14-3-3 dimers as adapters comes from studies of 14-3-3s interacting with the plant plasma-membrane proton ATPase and the plant toxin fusicoccin . It has been suggested that the 14-3-3 dimer binds the carboxy-terminal autoinhibitory (C-TA) domain of the ATPase in the presence of magnesium, creating a binding site where fusicoccin can then interact . Once fusicoccin is bound, the complex of 14-3-3 and the ATPase is stabilized and the C-TA domain is displaced, allowing the ATPase to become fully active.
Localization and function
In general, 14-3-3s are distributed widely throughout the cell, supporting the argument that they are involved in multiple protein-protein interactions in a plethora of biological roles. There is, however, some differential subcellular localization, suggesting an element of specialization among specific isoforms. Localization data have been collected for eight of the Arabidopsis isoforms using isoform-specific antibodies and fusions of 14-3-3 to green fluorescent protein (GFP; Table 1). These data do not necessarily exhaust all possible locations for each isoform; instead, they support the idea that certain isoforms are recruited to distinct subcellular locations. The localization of 14-3-3 κ and υ was studied using carboxy-terminal GFP fusions in transgenic Arabidopsis . Fusions of κ with GFP tended to localize to the plasma membrane, whereas υ-GFP fusions tended to be found in the cytosol . Additional data collected using microscopy and immuno-cytochemistry of total nuclear extracts showed that at least five different forms of 14-3-3s are found in the nucleus . Chloroplast stromal extracts screened with isoform-specific antibodies showed that 14-3-3 μ and ε (members of the epsilon group) and 14-3-3 υ and ν (members of the non-epsilon group) were the only 14-3-3s prominently located in the chloroplast . The presence of the two non-epsilon members 14-3-3 ν and υ in the chloroplast suggests that these proteins, although located on distinct branches of the phylogenetic tree, may share similar roles and cellular locations . 14-3-3 ε and μ were also found in starch grains ; ε has also been found at the nuclear envelope during 14-3-3-GFP studies. GFP-ω fusion studies showed that 14-3-3 nuclear localization is regulated by the cell-cycle ; generally, 14-3-3-ω-GFP fusions were excluded from the nucleus, but they accumulated in the nucleus just after nuclear division and then relocated back out of the nucleus just before completion of cytokinesis . In addition, a nuclear export signal was identified in the 14-3-3s of the fission yeast Schizosaccharomyces pombe that is required (in concert with the Crm1 nuclear export machinery) for the shuttling of the mitosis-inducing protein Cdc25 out of the nucleus following DNA damage . Nuclear shuttling has emerged as an important biological role for 14-3-3s , and the nuclear export signal (I/LxxxLxxxLxL) is highly conserved in the 133 full-length 14-3-3 sequences currently available (Figures 1 and 3).
The 14-3-3s are also differentially expressed among tissues and organs (Table 1). Arabidopsis 1ψ, λ, μ, and ε are found in the leaves; ψ and λ are also expressed in the stems and flowers , and χ is expressed in pollen grains and stigma papillar cells . The fact that 14-3-3s are differentially expressed in various tissues and differentially localized in subcellular compartments adds a layer of complexity to the examination and determination of biological roles for 14-3-3s, a complexity that must be reconciled with the highly conserved nature of 14-3-3-target interactions.
The identification of mutants and the use of transgenic organisms have provided some insight into some of the biological roles and locations of some 14-3-3s. For instance, a mutation in the RAD24 protein, one of the two 14-3-3s in S. pombe, reduces the yeast's ability to keep DNA damage in check . In Saccharomyces cerevisiae, disruption of the 14-3-3 genes BMH1 and BMH2 creates a lethal phenotype that can be rescued by introducing 14-3-3 isoforms from Arabidopsis, Dictyostelium discoideum, or Homo sapiens . The Leonardo (14-3-3 ζ) protein of Drosophila melanogaster is known to regulate presynaptic function, and its mutation results in the death of mature embryos . Two transgenic Arabidopsis lines, one carrying an antisense construct against 14-3-3 μ and another against 14-3-3 ε show dramatic increases in starch production in leaves .
Various 14-3-3s in a variety of species have been found to interact with proteins involved in signal transduction (such as Raf-1 ), apoptosis (such as the Bcl2-related protein Bad ), cell-cycle control (such as Cdc25 ), starch synthesis , nitrogen metabolism , and ATP regulation (reviewed in ).
Initially, discoveries of 14-3-3s were almost coincidental in nature; in many cases their identification was serendipitous after investigating other biochemical questions. Once it became established that these proteins were ubiquitous, research was directed toward identifying the number and sequences of isoforms present in different species as well as determining their functional diversity. As more genomes are sequenced, experimental tasks will move towards elucidation of general roles as well as investigation of isoform-specific roles in order to address the implications of 14-3-3 family diversity within organisms. Such studies are key to understanding the current conundrum: the conservation of 14-3-3s throughout eukaryotes suggests that some central biological roles might be served by any 14-3-3 protein, yet the diversity of 14-3-3 isoforms argues for a multitude of specific roles. Indeed, some combination of both concepts might be the case, with some roles being served by any isoform, and other roles requiring isoform-specific interactions. In any case, all the current data suggest that interactions involving 14-3-3 proteins are critical for the correct function of higher-order biological systems.
The presence of 14-3-3 proteins in most, if not all, eukaryotic cells, but not in any prokaryotic cells, offers an interesting opportunity to study the early evolutionary history of this protein family and the concomitant development of eukaryotic regulatory processes.
Moore B, Perez V: Specific acidic proteins of the nervous system. In In Physiological and Biochemical Aspects of Nervous Integration.. Edited by Carlson FD. Englewood Cliffs, NJ: Prentice-Hall; 1967:343-359.
The first identification of 14-3-3 proteins in mammal brains.
Ichimura T, Isobe T, Okuyama T, Yamauchi T, Fujisawa H: Brain 14-3-3 protein is an activator protein that activates tryptophan 5-monooxygenase and tyrosine 3-monooxygenase in the presence of Ca2+, calmodulin-dependent protein kinase II.
FEBS Lett 1987, 219:79-82.
This study was the first to assign a role to 14-3-3 proteins. It was shown that 14-3-3s activate tyrosine and tryptophan hydroxylases in a calcium- and kinase-dependent manner.PubMed Abstract | Publisher Full Text
Proc Natl Acad Sci USA 1992, 89:11490-11494.
The first identification of 14-3-3 proteins in Arabidopsis thaliana. They were found to be associated with G-box-binding factors.PubMed Abstract | Publisher Full Text | PubMed Central Full Text
Plant Cell 1992, 4:1295-1307.
The first identification of 14-3-3 proteins in maize.PubMed Abstract | Publisher Full Text
BioEssays 2001, 23:936-946.
A review discussing the roles of 14-3-3s in cell-cycle control, signal transduction, and apoptosis.PubMed Abstract | Publisher Full Text
Trends Plant Sci 1999, 4:367-371.
A review of the participation of 14-3-3s in plant regulatory events, including the regulation of plasma membrane H1-ATPase, nitrate reductase and sucrose phosphate synthase.PubMed Abstract | Publisher Full Text
Plant Physiol 2001, 126:35-38.
A review of Arabidopsis 14-3-3 gene structure and phylogeny.PubMed Abstract | Publisher Full Text
J Mol Evol 2000, 51:446-458.
This study describes the binding affinity of Arabidopsis 14-3-3 isoforms to a unique peptide and discusses the potential for functional specificity for these proteins across all multicellular species. A substantial family tree is also presented.PubMed Abstract | Publisher Full Text
Annu Rev Plant Physiol Plant Mol Biol 1996, 47:49-73.
A comprehensive review of the history, functions, cell-specific expression, evolution and structure of 14-3-3 proteins.Publisher Full Text
Cell 1996, 84:889-897.
This study was the first to show that 14-3-3 is a specific phosphoserine-binding protein.PubMed Abstract | Publisher Full Text
Cell 1997, 91:961-971.
Two different binding motifs were identified after screening mammalian and yeast peptide libraries with 14-3-3 proteins. Additionally, the crystal structure of 14-3-3 ζ bound to a phosphoserine target peptide was solved.PubMed Abstract | Publisher Full Text
Andrews RK, Harris SJ, McNally T, Berndt MC: Binding of purified 14-3-3 zeta signaling protein to discrete amino acid sequences within the cytoplasmic domain of the platelet membrane glycoprotein Ib-IX-V complex.
Biochemistry 1998, 37:638-647.
This study presents another non-phosphorylated target, GHSL, discovered via co-purification experiments.PubMed Abstract | Publisher Full Text
J Biol Chem 1998, 273:16305-16310.
14-3-3 crystal structures in complex with phosphorylated and unphosphorylated peptide were solved to determine the location of the binding site.PubMed Abstract | Publisher Full Text
Plant Cell 2002, 14:S339-S354.
A comprehensive review of the involvement of 14-3-3 proteins in signal transduction.PubMed Abstract | Publisher Full Text
Nature 1995, 376:191-194.
Crystal structure of the mammalian 14-3-3 ζ isoform.PubMed Abstract | Publisher Full Text
Nature 1995, 376:188-191.
Crystal structure of the mammalian 14-3-3 τ isoform.PubMed Abstract | Publisher Full Text
J Biol Chem 1997, 272:27281-27287.
Mutation of serine to alanine severely reduced the level of 14-3-3 and PTPH1 binding.PubMed Abstract | Publisher Full Text
J Biol Chem 1998, 273:16297-16304.
Mutation of hydrophobic residues on the 14-3-3 amphipathic groove shows that these residues are involved with binding the client protein. The mutated residues are conserved among isoforms, suggesting a general importance for ligand binding.PubMed Abstract | Publisher Full Text
Mol Cell Biol 1998, 18:5229-5238.
Overexpression of 14-3-3s causes increased phosphorylation of Raf-1. Similarly, removal of 14-3-3 reduces the kinase activity of Raf-1.PubMed Abstract | Publisher Full Text | PubMed Central Full Text
Annu Rev Pharmacol Toxicol 2000, 40:617-647.
Comprehensive review of 14-3-3-ligand interaction on a structural basis, a proposal of 14-3-3 function in several pathways, and a discussion of regulatory mechanisms.PubMed Abstract | Publisher Full Text
FEBS Lett 1995, 368:55-58.
Evidence that 14-3-3 proteins form both hetero- and homodimers.PubMed Abstract | Publisher Full Text
Rittinger K, Budman J, Xu J, Volinia S, Cantley LC, Smerdon SJ, Gamblin SJ, Yaffe MB: Structural analysis of 14-3-3 phosphopeptide complexes identifies a dual role for the nuclear export signal of 14-3-3 in ligand binding.
Mol Cell 1999, 4:153-166.
A nuclear export signal was identified in 14-3-3 proteins and shown to share topology with other signals recognized by the Crm1 nuclear export machinery.PubMed Abstract | Publisher Full Text
Plant J 1997, 12:1439-1445.
Scanning confocal microscopy and immunocytochemistry with mono-clonal antibodies against plant 14-3-3 proteins were used for localization studies.PubMed Abstract | Publisher Full Text
Sehnke PC, Henry R, Cline K, Ferl RJ: Interaction of a plant 14-3-3 protein with the signal peptide of a thylakoid-targeted chloroplast precursor protein and the presence of 14-3-3 isoforms in the chloroplast stroma.
Plant Physiol 2000, 122:235-242.
Immuno-electron microscopy of leaf tissue and western-blotting analysis of chloroplast fractions with monoclonal anti-14-3-3 antibodies localized 14-3-3 proteins to the chloroplast stroma and the stromal side of thylakoid membranes.PubMed Abstract | Publisher Full Text | PubMed Central Full Text
Plant Cell 1996, 8:1239-1248.
In situ hybridization showed that expression of χ (chi) mRNA was prominent in epidermal tissue of roots, petals, and sepals of flower buds, papillae cells of flowers, siliques, and endosperm of immature seeds. These results show plant 14-3-3 gene expression exhibits cell-and tissue-specific localization rivaling that observed for 14-3-3 proteins within the mammalian brain.PubMed Abstract | Publisher Full Text
Proc Natl Acad Sci USA 2001, 98:765-770.
14-3-3 proteins in Arabidopsis were shown to regulate starch synthesis through the use of two antisense constructs of 14-3-3 ε and 14-3-3 μ.PubMed Abstract | Publisher Full Text | PubMed Central Full Text
Proc Natl Acad Sci USA 2000, 97:3718-3723.
14-3-3-GFP fusions were observed in hypocotyl cells by confocal microscopy and showed localization to nuclei undergoing cytokinesis.PubMed Abstract | Publisher Full Text | PubMed Central Full Text
Science 1994, 265:533-535.
This investigation identified two 14-3-3 proteins, Rad24 and Rad25, that are involved in the DNA-damage checkpoint during mitosis.PubMed Abstract
Nature 1999, 397:172-175.
The Rad24 14-3-3 protein controls the intracellular distribution of Cdc25.PubMed Abstract | Publisher Full Text
Plant Physiol 1999, 120:1206.
An entry in the Plant Gene Register. Sequences were obtained from stem and flower tissues.
FEBS Lett 1996, 391:252-256.
In this exquisite study, the genes encoding two indigenous 14-3-3 iso-forms in bakers' the phenotypes were rescued by replacement of the genes with those of four of six Arabidopsis isoforms.PubMed Abstract | Publisher Full Text
Neuron 1997, 19:391-402.
Immunolocalization studies showed that the Leonardo 14-3-3 protein was expressed at synaptic connections enriched in presynaptic boutons of the neuromuscular junction. Null leonardo mutants die as mature embryos.PubMed Abstract | Publisher Full Text
Genes Dev 1997, 11:1132-1139.
This study discusses how 14-3-3 ε protein appears to function in multiple receptor tyrosine kinase signal transduction pathways.PubMed Abstract
Cell 1996, 87:619-628.
An example of how a 14-3-3 is involved with apoptosis. In the presence of a survival factor, Bad was phosphorylated on two serine residues embedded in 14-3-3 consensus binding sites. Only the non-phosphorylated Bad promotes cell death.PubMed Abstract | Publisher Full Text
Science 1997, 277:1501-1505.
An example of 14-3-3s involved in cell-cycle control. A mutation in human Cdc25c prevented the phosphorylation of Ser216, thus preventing 14-3-3 binding. Conditional overexpression of this mutant perturbed mitotic timing and allowed cells to escape the G2 checkpoint arrest induced by either unreplicated DNA or radiation-induced damage.PubMed Abstract | Publisher Full Text
Bachmann M, Huber JL, Athwal GS, Wu K, Ferl RJ, Huber SC: 14-3-3 proteins associate with the regulatory phosphoryla-tion site of spinach leaf nitrate reductase in an isoform-specific manner and reduce dephosphorylation of Ser-543 by endogenous protein phosphatases.
FEBS Lett 1996, 398:26-30.
14-3-3 proteins were shown to regulate nitrate reductase by binding a synthetic phosphonitrate reductase peptide.PubMed Abstract | Publisher Full Text
Access to the GenBank sequence database.
Our lab's website contains information on 14-3-3s and related research.
Thompson JD, Higgins DG, Gibson TJ: CLUSTAL W: improving the sensitivity of progressive multiple sequence alignment through sequence weighting, position-specific gap penalties and weight matrix choice.
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CLUSTAL W is a common method used to generate multiple sequence alignments.PubMed Abstract
Mol Biol Evol 1987, 4:406-425.
The neighbor-joining method is a statistical method used to generate possible phylogenetic trees.PubMed Abstract | Publisher Full Text