**2.3 Yeast MAPK cascades**

Five MAPK pathways have been well characterized in the budding yeast, *S. cerevisiae*. In vegetative cells, the four MAPKs Fus3, Kss1, Hog1, and Slt2/Mpk1 are involved in the mating-pheromone response, the filamentous-invasion pathway, the high osmolarity growth, and the cell integrity pathway, respectively. The fifth MAPK, Smk1, is believed to play a role in spore wall assembly [16, 17].

Haploid yeast cells sense the reciprocal mating pheromones (α-factor or a-factor) through Ste2 and Ste3 GPCRs. The signal is then transmitted by GTPases to the p21-activated kinase (PAK)-like kinase Ste20, the MAPK scaffold Ste5, Cdc42, a guanine-nucleotide exchange factor (GEF), and Far1. Ste5 signals and serves as a scaffold that links the MAP4K and the MAPK signalosome (Ste11 → Ste7 → Fus3; the latter is the ERK1 homolog) [18].

The high osmolarity glycerol (Hog1) MAPK, the yeast homolog of p38, is activated in response to osmotic stress as a consequence of signaling elicited from two upstream-independent mechanisms (Sln1/Sho1). The Sln1 sensor is the primary osmosensor and is a complex variation of the well-known bacterial two-component system. Upon osmostress, inactivation of the transmembrane histidine kinase Sln1 leads to the derepression and activation of the MAP3K (Ssk2/22) via Ypd1/Skk1. The Sho1 osmosensing branch is mediated by mucin-like proteins (Hkr1 and Msb2) and ultimately activates the MAP3K Ste11 through the integral transmembrane

protein Opy2, the GTPase Cdc42 and the MAP4K Ste20. These two osmosensing branches converge at the MAP2K (Pbs2) that acts as a scaffold protein for phosphorylation of the MAPK Hog1 [19].

The filamentous/invasive growth pathway leads to the activation of Kss1 (an ortholog of mammalian Erk2) under nutrient limiting conditions and, to a much lesser extent, to pheromone stimulation. Remarkably, it relies on proteins involved in the HOG pathway and the pheromone pathway (Mep2, Gpr1, Msb2, Sho1, Ste20, Ste11, and Ste7). In this case, specific activation of Kss1 is achieved by the absence of the Ste5 scaffold that liberates Ste7 allowing its interaction with Kss1 [20].

Cell wall instability is sensed through the cell wall integrity pathway (CWI) (Mpk1 MAPK) and is detected by five mechanosensors (Wsc1–3, Mid2, and Mtl1) that interact with the guanine nucleotide exchange factor (GED) Rom2 to activate Rho1 GTPase leading to protein kinase C (Pkc1) phosphorylation. Yeast Pkc1 serves as a MAP4K that phosphorylates the MAP3K Bck1, which leads to activation of Mpk1 through activation of the redundant MAP2K (Mkk1/2). Despite the absence of a cell wall in higher eukaryotes, mammalian ERK5 has been characterized as a functional ortholog of the CWI pathway [21].

Finally, the meiosis-specific MAPK Smk1 controls the postmeiotic program in diploid cells subjected to nutrient starvation. Activation of Smk1 differs from activation of MAPKs in the classical three-tiered MAPK cascade in which a CDK-activating kinase (CAK1) phosphorylates Smk1 and induces its auto-phosphorylation [22].

#### **2.4 Dynamics of signal transduction**

According to the nature of its input signal, MAPK activation can range from minutes (transient) to hours (sustained). The dynamics of MAPK activation results from the interplay between the extracellular environment and a myriad of intracellular feedforward/feedback regulators that give rise to cell fate decisions during cancer progression or development. For example, pulses of or continuous high EGF administration induce transient ERK activation and cell proliferation in rat adrenal cells, whereas repeated pulses of low EGF induce ERK-mediated differentiation into sympathetic-like neurons [23]. Similarly, different dynamics of JNK can generate opposing behaviors as persistent JNK activation has been shown to trigger apoptosis while its transient activation promotes cell survival [24]. Despite different signaling dynamics can determine cell fate, the underlying molecular mechanisms are not well understood.

#### **2.5 Output responses to MAPK activation**

The first response to extracellular insults is the immediate arrest of cell growth and hence a blockage of or a delay in cell cycle progression. Once activated, the different MAPKs phosphorylate a large number of substrates that distribute over many cellular compartments. In general terms, the ERK1/2 pathway is mainly associated with the promotion of growth in most cell types and is often linked with differentiation processes, although it can occasionally suppress cell survival [25]. Similarly, ERK5 also promotes proliferation during normal cell growth and differentiation [3]. On the other hand, JNK and p38 pathways have a well-established role in apoptosis, although they have also been shown to contribute to survival, immunity, development, and differentiation [4, 26–29]. One of the main mechanisms by which MAPKs modulate the abovementioned cellular processes is by controlling gene expression, mainly through regulation of the transcriptional machinery, chromatin structure, and post-transcriptional modifications.

**25**

**Figure 2.**

*Shaping the Transcriptional Landscape through MAPK Signaling*

**3. Nuclear localization and function of MAPKs**

In resting cells, MAPK components are usually located in the cytoplasm through their interaction with different anchor proteins, scaffolds, or phosphatases. Upon stimulation, MAPK signaling cascades rapidly transmit information into the nucleus to ensure the appropriate transcriptional response (**Figure 2A**). Across eukaryotes, this process is often initiated by transient accumulation of the MAPKs within the nucleus. The duration and the type of stimuli affect the nuclear localization of MAPK signaling proteins and play an important role in determination of the transcriptional output. Translocation of MAPK molecules requires specialized transport elements to travel through the nuclear pore complex (NPC).

*MAPK regulatory roles on gene expression: From transcription initiation to translation. (***A***) Activated MAPK is released from its cytoplasmic anchor and translocated to the nucleus. (***B***) From top and clockwise, MAPK regulation on different targets is represented by a black arrow; MAPKs are known to activate transcription factors (TFs) through phosphorylation and to recruit PolII to initiate transcription. Moreover, MAPKs also target several chromatin remodelers (Ch Rem) and histone modifiers (Hist mod) to regulate chromatin structure and histone eviction. MAPK interacts with transcription elongating (TEF) and termination factors to enhance transcription rate. mRNA is shown as a green line with a 5*′ *cap (green dot) and the polyA at the 3*′ *end. MAPKs also regulate several stabilizing RNA-binding proteins (RBPs), target miRNA processing through the microprocessor complex (MC), nuclear exporters, and splicing factors. Finally, MAPKs are also known to regulate translation elongation initiation factors (eIFs) to stimulate rapid mRNA translation. Overall, all these* 

*mechanisms aim to promote a rapid and efficient response for maximal cell adaptation.*

*DOI: http://dx.doi.org/10.5772/intechopen.80634*

*Gene Expression and Control*

Kss1 [20].

phorylation of the MAPK Hog1 [19].

functional ortholog of the CWI pathway [21].

**2.4 Dynamics of signal transduction**

**2.5 Output responses to MAPK activation**

structure, and post-transcriptional modifications.

protein Opy2, the GTPase Cdc42 and the MAP4K Ste20. These two osmosensing branches converge at the MAP2K (Pbs2) that acts as a scaffold protein for phos-

The filamentous/invasive growth pathway leads to the activation of Kss1 (an ortholog of mammalian Erk2) under nutrient limiting conditions and, to a much lesser extent, to pheromone stimulation. Remarkably, it relies on proteins involved in the HOG pathway and the pheromone pathway (Mep2, Gpr1, Msb2, Sho1, Ste20, Ste11, and Ste7). In this case, specific activation of Kss1 is achieved by the absence of the Ste5 scaffold that liberates Ste7 allowing its interaction with

Cell wall instability is sensed through the cell wall integrity pathway (CWI) (Mpk1 MAPK) and is detected by five mechanosensors (Wsc1–3, Mid2, and Mtl1) that interact with the guanine nucleotide exchange factor (GED) Rom2 to activate Rho1 GTPase leading to protein kinase C (Pkc1) phosphorylation. Yeast Pkc1 serves as a MAP4K that phosphorylates the MAP3K Bck1, which leads to activation of Mpk1 through activation of the redundant MAP2K (Mkk1/2). Despite the absence of a cell wall in higher eukaryotes, mammalian ERK5 has been characterized as a

Finally, the meiosis-specific MAPK Smk1 controls the postmeiotic program in diploid cells subjected to nutrient starvation. Activation of Smk1 differs from activation of MAPKs in the classical three-tiered MAPK cascade in which a CDK-activating kinase (CAK1) phosphorylates Smk1 and induces its auto-phosphorylation [22].

According to the nature of its input signal, MAPK activation can range from minutes (transient) to hours (sustained). The dynamics of MAPK activation results from the interplay between the extracellular environment and a myriad of intracellular feedforward/feedback regulators that give rise to cell fate decisions during cancer progression or development. For example, pulses of or continuous high EGF administration induce transient ERK activation and cell proliferation in rat adrenal cells, whereas repeated pulses of low EGF induce ERK-mediated differentiation into sympathetic-like neurons [23]. Similarly, different dynamics of JNK can generate opposing behaviors as persistent JNK activation has been shown to trigger apoptosis while its transient activation promotes cell survival [24]. Despite different signaling dynamics can determine cell fate, the underlying molecular mechanisms are not

The first response to extracellular insults is the immediate arrest of cell growth and hence a blockage of or a delay in cell cycle progression. Once activated, the different MAPKs phosphorylate a large number of substrates that distribute over many cellular compartments. In general terms, the ERK1/2 pathway is mainly associated with the promotion of growth in most cell types and is often linked with differentiation processes, although it can occasionally suppress cell survival [25]. Similarly, ERK5 also promotes proliferation during normal cell growth and differentiation [3]. On the other hand, JNK and p38 pathways have a well-established role in apoptosis, although they have also been shown to contribute to survival, immunity, development, and differentiation [4, 26–29]. One of the main mechanisms by which MAPKs modulate the abovementioned cellular processes is by controlling gene expression, mainly through regulation of the transcriptional machinery, chromatin

**24**

well understood.
