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Mitogen-Activated Protein Kinases (MAPKs)

Mitogen-Activated Protein Kinases (MAPKs) play a crucial role in cellular signaling, affecting proliferation, differentiation, and apoptosis. They are activated through a cascade involving phosphorylation by MAP kinase kinases and inactivated by MAP kinase phosphatases. This text delves into the MAPKs' functions, their classification into ERKs, JNKs, and p38 MAPKs, and the unique pathways of atypical MAPKs.

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1

The ______ kinase group, which includes MAPKs, is found only in ______ cells and is essential for many cellular functions.

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CMGC eukaryotic

2

Research on MAPKs started with the discovery of ______ in mammals, and these kinases are known to play a role in ______ stress responses in plants.

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ERK1 abiotic

3

ERK activation stimuli

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ERKs activated by growth factors, mitogens.

4

JNK and p38 MAPK activation stimuli

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JNKs, p38 MAPKs respond to stressors, inflammatory cytokines.

5

MAPKs role in cellular response

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MAPKs transduce external signals into cellular responses, indicating functional diversity.

6

Classical ______ are initially inactive and need a series of phosphorylation to become active.

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MAPKs

7

The ______ group of protein kinases facilitates the phosphorylation that leads to MAPKs' activation.

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STE

8

The activation loop of classical MAPKs contains a ______ motif, essential for kinase domain activation.

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TxY

9

In classical MAPKs, both ______ and ______ residues in the TxY motif must be phosphorylated for activation.

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threonine tyrosine

10

MAP kinase kinase kinases, also known as ______, activate MKKs in the signaling cascade.

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MAP3Ks

11

The signaling cascade from the cell membrane to the nucleus involves precise transmission through a ______ structure.

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hierarchical

12

Characteristic phosphorylation of atypical MAPKs

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Atypical MAPKs require phosphorylation at a single residue within their activation loops.

13

ERK3 and ERK4 activation

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ERK3 and ERK4 are phosphorylated directly by PAK family kinases, bypassing the need for a three-tiered cascade.

14

Signaling architecture of atypical MAPKs

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Atypical MAPKs often function within simpler two-tiered pathways, unlike the three-tiered systems of classical MAPKs.

15

The inactivation of ______ is as crucial as their activation for precise cellular signaling control.

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MAPKs

16

MKPs can remove phosphate groups from both ______ and ______ residues, ending MAPK activity.

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phosphotyrosine phosphothreonine

17

Certain ______-specific phosphatases also contribute to the inactivation of MAPKs, ensuring tight regulation of cellular signaling.

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tyrosine

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Overview of Mitogen-Activated Protein Kinases (MAPKs)

Mitogen-Activated Protein Kinases (MAPKs) are a pivotal category of protein kinases that specifically phosphorylate the amino acids serine and threonine. These enzymes are integral to the cellular response to diverse stimuli, including mitogens, osmotic stress, heat shock, and proinflammatory cytokines. MAPKs orchestrate a multitude of cellular processes, such as proliferation, gene expression, differentiation, cell cycle progression, survival, and programmed cell death (apoptosis). Found exclusively in eukaryotic cells, MAPKs are part of the CMGC (Cyclin-dependent kinases, MAPK, GSK3, and CLK kinases) kinase group. The exploration of MAPKs began with the identification of ERK1 in mammals, and further research has uncovered their role in abiotic stress responses, with plants demonstrating a particularly extensive array of MAPK genes to cope with environmental challenges.
Three-dimensional molecular model of a protein with colored spheres for atoms and sticks for bonds on a blue-white gradient background.

Classification and Subfamilies of MAPKs

Within the mammalian MAPK family, there are three principal subfamilies: Extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38 mitogen-activated protein kinases (p38 MAPKs). Each subfamily is triggered by distinct stimuli; ERKs are commonly activated by growth factors and mitogens, whereas JNKs and p38 MAPKs respond to cellular stressors and inflammatory cytokines. This functional diversity is indicative of the specialized roles MAPKs fulfill in transducing a wide array of external signals into appropriate cellular responses.

Activation and Function of Classical MAPKs

Classical MAPKs are initially in an inactive state and require sequential phosphorylation events to become catalytically active. These phosphorylation events are mediated by the STE group of protein kinases, which promote a conformational change in the MAPK through phosphorylation. The activation loop of classical MAPKs features a TxY motif, where both the threonine and tyrosine residues must be phosphorylated to activate the kinase domain. This phosphorylation is executed by MAP kinase kinases (MKKs or MAP2Ks), which are in turn activated by MAP kinase kinase kinases (MAP3Ks). This hierarchical signaling cascade ensures the precise transmission of signals from the cell membrane to the nucleus or other intracellular destinations.

Atypical MAPKs and Their Unique Activation Pathways

Beyond the classical MAPKs, there exists a group of atypical MAPKs that are characterized by distinct activation mechanisms and signaling architectures. These atypical kinases do not require the dual phosphorylation characteristic of classical MAPKs and often function within simpler two-tiered pathways. For instance, ERK3 and ERK4 are phosphorylated directly by certain members of the PAK family of kinases, which are akin to MAP3 kinases. Atypical MAPKs necessitate phosphorylation at a single residue within their activation loops, underscoring the evolutionary adaptability and diversity of the MAPK enzyme family.

Inactivation of MAPKs by Phosphatases

The deactivation of MAPKs is as critical as their activation for maintaining precise control over cellular signaling. This inactivation is predominantly facilitated by MAP kinase phosphatases (MKPs), which are part of the dual-specificity phosphatases (DUSPs) family. MKPs are capable of dephosphorylating both phosphotyrosine and phosphothreonine residues, thereby terminating MAPK activity. In addition, certain tyrosine-specific phosphatases also play a role in MAPK inactivation, ensuring that cellular signaling is tightly regulated and concluded when no longer needed.