In this review we highlight the advances achieved in the investigation of the role of 14-3-3 proteins in hormone signaling, biosynthesis, and transport. 14-3-3 proteins are a family of conserved molecules that target a number of protein clients through their ability to recognize well-defined phosphorylated motifs. As a result, they regulate several cellular processes, ranging from metabolism to transport, growth, development, and stress response. High-throughput proteomic data and two-hybrid screen demonstrate that 14-3-3 proteins physically interact with many protein clients involved in the biosynthesis or signaling pathways of the main plant hormones, while increasing functional evidence indicates that 14-3-3-target interactions play pivotal regulatory roles. These advances provide a framework of our understanding of plant hormone action, suggesting that 14-3-3 proteins act as hubs of a cellular web encompassing different signaling pathways, transducing and integrating diverse hormone signals in the regulation of physiological processes.

Fusicoccin is the α glucoside of a carbotricyclic diterpene, produced by the fungus Phomopsis amygdali (previously classified as Fusicoccum amygdali), the causal agent of almond and peach canker disease. A great interest in this molecule started when it was discovered that it brought about an irreversible stomata opening of higher plants, thereby inducing the wilting of their leaves. Since then, several studies were carried out to elucidate its biological activity, biosynthesis, structure, structure-activity relationships and mode of action. After sixty years of research and more than 1800 published articles, FC is still the most studied phytotoxin and one of the few whose mechanism of action has been elucidated in detail. The ability of FC to stimulate several fundamental plant processes depends on its ability to activate the plasma membrane H⁺-ATPase, induced by eliciting the association of 14-3-3 proteins, a class of regulatory molecules widespread in eukaryotes. This discovery renewed interest in FC and prompted more recent studies aimed to ascertain the ability of the toxin to influence the interaction between 14-3-3 proteins and their numerous client proteins in animals, involved in the regulation of basic cellular processes and in the etiology of different diseases, including cancer. This review covers the different aspects of FC research partially treated in different previous reviews, starting from its discovery in 1964, with the aim to outline the extraordinary pathway which led this very uncommon diterpenoid to evolve from a phytotoxin into a tool in plant physiology and eventually into a 14-3-3-targeted drug.

Modulation of the interaction of regulatory 14-3-3 proteins to their physiological partners through small cell-permeant molecules is a promising strategy to control cellular processes where 14-3-3s are engaged. Here, we show that the fungal phytotoxin fusicoccin (FC), known to stabilize 14-3-3 association to the plant plasma membrane H⁺-ATPase, is able to stabilize 14-3-3 interaction to several client proteins with a mode III binding motif. Isothermal titration calorimetry analysis of the interaction between 14-3-3s and different peptides reproducing a mode III binding site demonstrated the FC ability to stimulate 14-3-3 the association. Moreover, molecular docking studies provided the structural rationale for the differential FC effect, which exclusively depends on the biochemical properties of the residue in peptide C-terminal position. Our study proposes FC as a promising tool to control cellular processes regulated by 14-3-3 proteins, opening new perspectives on its potential pharmacological applications. © 2014 IUBMB Life, 66(1):52–62, 2014

Ophiobolin A, a fungal toxin from Bipolaris species known to affect different cellular processes in plants, has recently been shown to have anti-cancer activity in mammalian cells. In the present study, we investigated the anti-proliferative effect of Ophiobolin A on human melanoma A375 and CHL-1 cell lines. This cellular model was chosen because of the incidence of melanoma malignant tumor on human population and its resistance to chemical treatments. Ophyobolin A strongly reduced cell viability of melanoma cells by affecting mitochondrial functionality. The toxin induced depolarization of mitochondrial membrane potential, reactive oxygen species production and mitochondrial network fragmentation, leading to autophagy induction and ultimately resulting in cell death by activation of the mitochondrial pathway of apoptosis. Finally, a comparative proteomic investigation on A375 cells allowed to identify several Ophiobolin A down-regulated proteins, which are involved in fundamental processes for cell homeostasis and viability.

The fungal toxin fusicoccin induces plant wilting by affecting ion transport across the plasma membrane of plant cell. The activity of this toxin is so far unknown in humans. In the present study we show that fusicoccin is able to affect the platelet aggregation process. The toxin associates with platelet intracellular binding sites and induces aggregation in platelet-rich plasma in a dose-dependent manner. We identified the adhesion receptor glycoprotein Ib-IX-V as fusicoccin target. The toxin promotes the binding of the regulatory 14-3-3 proteins to glycoprotein Ibα and hampers that to glycoprotein Ibβ subunit. As a result, platelet adhesion to von Willebrand factor is stimulated, leading to platelet spreading and integrin αIIbβ3 activation. We anticipate the present study to be a starting point for future therapeutic use of fusicoccin in genetic bleeding diseases characterized by qualitative or quantitative abnormalities of the platelet membrane-adhesion receptors. Furthermore, the present study also sets the stage for future work to determine the potential pharmacological application of fusicoccin as a drug directed to other 14-3-3–target complexes.

Phosphatidic acid is a phospholipid second messenger implicated in various cellular processes in eukaryotes. In plants, production of phosphatidic acid is triggered in response to a number of biotic and abiotic stresses. Here, we show that phosphatidic acid binds to 14‐3‐3 proteins, a family of regulatory proteins which bind client proteins in a phosphorylation‐dependent manner. Binding of phosphatidic acid involves the same 14‐3‐3 region engaged in protein target binding. Consequently, micromolar phosphatidic acid concentrations significantly hamper the interaction of 14‐3‐3 proteins with the plasma membrane H⁺‐ATPase, a well characterized plant 14‐3‐3 target, thus inhibiting the phosphohydrolitic enzyme activity. Moreover, the proton pump is inhibited when endogenous PA production is triggered by phospholipase D and the G protein agonist mastoparan‐7. Hence, our data propose a possible mechanism involving PA that regulates 14‐3‐3‐mediated cellular processes in response to stress. © 2012 IUBMB IUBMB Life, 64(8): 710–716, 2012