Tyrosine phosphorylation in plant cell signaling.

R protein phosphorylation is the most common mechanism for cellular regulation in eukaryotic systems. Studies have demonstrated that serine threonine phosphorylation plays a key role in the regulation of plant growth and development. On the other hand, tyrosine phosphorylation, despite its overwhelming importance in animals, has been largely neglected because a typical tyrosine kinase was not found in plants. Recent studies have characterized several protein tyrosine phosphatases (PTPs) from Arabidopsis and other species (1–3). Furthermore, a diverse group of about 20 genes encoding putative tyrosine phosphatases have been identified from the Arabidopsis genome, implying that tyrosine phosphorylation and dephosphorylation may serve important functions in plant biology. It is timely and exciting to learn in this issue of PNAS that tyrosine phosphatase activity is involved in the regulation of stomatal movement, a highly regulated process pivotal for plant survival (4). Finding a role for tyrosine phosphatases in stomatal regulation provides critical evidence that tyrosine phosphatases not only exist but also play an important role in higher plants. This work, together with those of others, begins to modify the earlier tenets on tyrosine phosphorylation in plant cell signaling and regulation. Considerable differences are apparent when animals and plants are compared regarding protein phosphorylation in signal transduction. It is well established that both Ser Thr and tyrosine phosphorylation play pivotal roles in cell signaling in animals. In particular, protein-tyrosine phosphorylation serves as a common mechanism by which growth factors and cytokines regulate cellular proliferation and differentiation in animals (5–7). The level of tyrosine phosphorylation in normal cells is determined by the balanced activity of protein tyrosine kinases (PTKs) and PTPs. Even the slightest tipping of this balance may result in cancer or abnormal cell death. As a result, a typical animal cell expresses a large number of PTKs and PTPs to fine-tune cellular proliferation differentiation. In contrast, available information indicates that plants produce a large number of Ser Thr kinases phosphatases that function in plant signal transduction and regulation, but a typical tyrosine kinase has not yet been characterized from a plant species. Interestingly, bona fide tyrosine-specific protein phosphatases do exist in plants (1, 3). In addition, plants, like animals, produce a large number of protein phosphatases that dephosphorylate phosphoserine threonine in addition to phosphotyrosine, so-called dual-specificity protein (tyrosine) phosphatases (DsPTPs or DSPs) (2). The DsPTPs have been shown to regulate the mitogen-activated protein kinases (MAPKs) in a variety of signal transduction pathways in both animals and yeast. Because a large number of MAPKs have also been found in plants, PTP DsPTP regulation of MAPKs could become a common ground where tyrosine phosphorylation dephosphorylation regulates cellular activities in the different eukaryotic systems. Recent studies have confirmed that this is indeed the case. MAPKs are activated after mitogen stimulation or environmental stress in mammalian cells (8). At the molecular level, MAPK activation relies on phosphorylation of both tyrosine and threonine residues. A variety of DsPTPs exhibit activity toward activated MAPK isoforms both in vitro and in vivo (9). In each instance, dephosphorylation of a MAPK by a DsPTP leads to loss of kinase activity. It is interesting that multiple DsPTPs can be found in a given cell type as are multiple MAPKs. Studies have shown that each isoform of DsPTPs may dephosphorylate and regulate only one or two MAPKs, a substrate specificity that is closely related to the function of different PTPs, and therefore are referred to as MKPs (for MAP kinase phosphatases) (9). Almost all signaling pathways are turned on transiently and need to be turned off rather quickly after the activation. This is reflected by the activation pattern of MAPKs as well. A signal activates a MAPK rapidly (often reaching peak activity within a few minutes), and the MAPK activity subsequently returns to the basal level to quench the signaling process. It should be emphasized that such transient on-and-off switching is very important for the physiological process regulated by MAPKs. Prolonged or constant activation of a MAPK cascade can have detrimental consequence to the cell as best illustrated by tumorigenesis in mammalian cells that have MAPK constantly on (10). In budding yeast, the Hog1 pathway is required for osmotic stress tolerance, yet a constant MAPK activity of Hog1 actually renders yeast hypersensitive to the stress condition (11). Needless to say, protein phosphatases, especially PTPs and DsPTPs, play a critical role in turning off the activity of MAPKs and are essential for keeping the precise kinetic pattern of MAPK activation and inactivation. Perhaps the first blow to the current paradigm that tyrosine phosphorylation is not important in plant cells comes from the finding of MAPK cascades in numerous signal transduction pathways in plants (reviewed in refs. 12–15). Included are those pathways responsible for transmitting biotic and abiotic stress and hormonal signals. Consistent with the diversity of MAPK functions, a large number of genes in the Arabidopsis genome have been identified to encode components in the MAPK cascades including MAPKs (about 20 genes), MAPKKs (10 genes), and MAPKKKs ( 25 genes) (14, 15). A unique feature of MAPK activation in animal and yeast cells is dual phosphorylation at a closely spaced pair of threonine and tyrosine. Earlier studies have shown that activation of plant MAPKs also correlates with phosphorylation of tyrosine residue(s). A detailed biochemical analysis confirms that a plant MAPK (AtMPK4) is phosphorylated at a tyrosine residue during activation process (16). Dephosphorylation of the phosphotyrosine by AtPTP1, a tyrosine-specific PTP from Arabidopsis (1), resulted in the loss of MAPK activity. This study demonstrates