Chemistry Reference
In-Depth Information
2.4. Role of phosphoproteins in stress signaling
By controlling the phosphorylation status of other proteins, protein kinases and
phosphatases play a fundamental role in coordinating the activity of many known signal
transduction pathways. For many signal pathways not only in abiotic stress field, protein
reversible phosphorylation is the major player in relaying signals. And during this
significant process, we highlight the functions of protein kinases and protein phosphatases
who are enzymes to catalyze these reversible phosphorylation processes. And they are
divided into several categories according to their structure or functional characteristics. And
in the following part, we will give a general idea to the readers about their central role in
signal transduction in abiotic stress aspect.
2.4.1. MAPK
Obviously, we have seen MAPK many times in formal description in this chapter. Even if it
is not found in plants, the mitogen activated protein kinase (MAPK) cascades are known to
be involved in plant abiotic stress responses acting as intracellular signal modules that
mediate signal transduction from the cell surface to the nucleus. The reason to mention it
here is that phosphorylation plays a central role in the progression of the signal through the
MAPK cascade. Moreover MAPK cascades, the conserved signaling modules found in all
eukaryotes, are fundamental in transducing environmental and developmental cues into
intracellular responses bringing changes in cellular organization or gene expression. The
simplest constitution of a MAPK cascade contains MAP kinase kinase kinases
(MAP3Ks/MAPKKKs/MEKKs), MAP kinase kinases (MAP2Ks/MAPKKs/MEKs/MKKs) and
MAP kinases (MAPKs/MPKs) (Mishra NS et al., 2006). And when under stress, stimulated
plasma membrane will activate MAP3Ks or MAP kinase kinase kinase kinases (MAP4Ks),
who may be the adapters to link upstream signaling steps to the core MAPK cascades (Dan I
et al., 2001). Following that, MAP3Ks will phosphorylate two amino acids in the S/T-X3-5-
S/T motif of the MAP2K activation loop. Then MAP2Ks phosphorylate MAPKs on threonine
and tyrosine residues at a conserved T-X-Y motif at the active site. When signals come to
MAPKs, further phosphorylation will tag on a wide range of substrates involving other
kinases, cytoskeleton-associated proteins, and/or transcription factors. As for formation and
integrity of a specific MAPK cascade, scaffold proteins take control over it (Whitmarsh AJ et
al., 1998). And after signaling completed, MKPs (MAPK phosphatases) take the responsibility
to shut the pathway down. Generally, the whole cascade is regulated by various mechanisms,
including not only transcriptional and translational regulation but through post-
transcriptional regulation such as protein-protein interactions (Rodriguez MC et al., 2010).
Thanks to traditional genetic and biochemical methods and lots of excellent research efforts,
we can conclude that MAP3K/MAP2K/MAPK signaling modules show overlapping roles in
controlling diverse cellular functions by forming complex interconnected networks within
cells. These include cell division, development, hormone signaling and synthesis, and
response to abiotic stress (high and low temperature, drought and high and low osmolarity,
