Dual specificity mitogen-activated protein kinase kinase 7, also known as MAP kinase kinase 7 or MKK7, is an

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

that in humans is encoded by the ''MAP2K7''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. This protein is a member of the

mitogen-activated protein kinase kinase Mitogen-activated protein kinase kinase (also known as MAP2K, MEK, MAPKK) is a dual-specificity kinase enzyme which phosphorylates mitogen-activated protein kinase (MAPK). MAP2K is classified as . There are seven genes: * (a.k.a. MEK1) * (a ...

family. The MKK7 protein exists as six different

isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...

with three possible N-termini (α, β, and γ isoforms) and two possible

C-termini The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When ...

(1 and 2 isoforms). MKK7 is involved in

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

mediating the cell responses to proinflammatory

cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...

s, and environmental stresses. This

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

specifically activates

MAPK8 Mitogen-activated protein kinase 8 (also known as JNK1) is a ubiquitous enzyme that in humans is encoded by the ''MAPK8'' gene. Function The protein encoded by this gene is a member of the MAP kinase and JNK family. MAP kinases act as an int ...

/JNK1 and MAPK9/JNK2, and this kinase itself is

phosphorylated In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...

and activated by MAP kinase kinase kinases including

MAP3K1 Mitogen-activated protein kinase kinase kinase 1 (MAP3K1) is a signal transduction enzyme that in humans is encoded by the autosomal ''MAP3K1'' gene. Function MAP3K1 (or MEKK1) is a serine/threonine kinase and ubiquitin ligase that performs a ...

/MEKK1, MAP3K2/MEKK2, MAP3K3/MEKK5, and MAP4K2/GCK. MKK7 is ubiquitously expressed in all tissue. However, it displays a higher level of expression in skeletal muscle. Multiple alternatively spliced transcript variants encoding distinct isoforms have been found.

Nomenclature

MAP2K7 is also known as: * MKK7 * JNK-activated kinase 2 * MAPK/ERK kinase 7 (MEK7) * Stress-activated protein kinase kinase 4 (SAPK kinase 4, SAPKK4) * c-Jun N-terminal kinase kinase 2 (JNK kinase 2, JNKK2) * Stress-activated / extracellular signal-regulated protein kinase kinase 2 (SEK2)

Isoforms

The murine MKK7 protein is encoded by 14

exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence ...

s which can be alternatively spliced to yield a group of protein

s. This results in six

isoform A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...

s with three possible N-termini (α, β, and γ isoforms) and two possible C-termini (1 and 2 isoforms). The molecular mass of the isoforms spans from 38 to 52

kDa The dalton or unified atomic mass unit (symbols: Da or u, respectively) is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted f ...

, with between 345 and 467 amino acids. The physiological relevance of the different MKK7 isoforms is still unclear. Evidence shows that the MKK7α, which lacks an NH₂-terminal extension, shows a lower basal activity in binding

JNK c-Jun N-terminal kinases (JNKs), were originally identified as kinases that bind and phosphorylate c-Jun on Ser-63 and Ser-73 within its transcriptional activation domain. They belong to the mitogen-activated protein kinase family, and are r ...

compared to the MKKβ and γ isoforms. The increased basal activity in the β and γ isoforms can be due to the three D-motifs present in the N-terminus of these isoforms.

Structure and function

D-motifs

MKK7 has three conserved D-motifs (MAPK-recruiting

short linear motif In molecular biology short linear motifs (SLiMs), linear motifs or minimotifs are short stretches of protein primary structure, protein sequence that mediate protein–protein interaction. The first definition was given by Tim Hunt: "The sequences ...

s) on its intrinsically disordered

N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...

. D-motifs typically consist of a cluster of positively charged

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s followed by alternating hydrophobic amino acids. D-motifs are strictly required for the recruitment of MAPKK substrates, such as JNK. The kinase domains of MAPKs contain certain surface features, such as the so-called common docking (CD) region, alongside the docking (D) groove, that specifically recognize their cognate D-motifs. The D-motifs found in MKK7 are highly specific for JNKs, but have a relatively low binding affinity. It was suggested that the motifs of MKK7 can synergize with each other to provide an efficient substrate phosphorylation It has been shown that all three D-motifs are necessary for correct JNK1:MKK7 complex formations, and for the phosphorylation and activation of JNK1 by MKK7.

DVD region

A special extension to the C-terminal kinase domain core, the so-called "Domain for Versatile Docking" (DVD) is a region found in MKK7 as in most known MAP2Ks,. The DVD region is a stable, mostly helical fold of roughly 20 amino acids, that adds onto the back side of the catalytic core of the MAP2K kinase domains. This domain extension is both required for the specific binding to, and activation of MKK7 by respective upstream MAPKKKs. Other mitogen activated protein kinase kinases also require the DVD region (in addition to various other non-canonical elements of their kinase domains, like the "MKK1/2-loop") to be able to discriminate against the various MAPKKK upstream. These special MAPKK:MAPKKK kinase-domain/kinase-domain interactions facilitate the phosphorylation of MKK7. In addition to the activation of MKK7, binding to the DVD region may also affect the MKK7

activation loop In molecular biology, an intrinsically disordered protein (IDP) is a protein that lacks a fixed or ordered three-dimensional structure, typically in the absence of its macromolecular interaction partners, such as other proteins or RNA. IDPs ran ...

in such a way that the Ser and Thr of the S-K-A-K-T motif become accessible for

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

Kinase domain

The MKK7 contains one kinase domain. The direct MKK7:MAPKKK interaction (using the DVD region), facilitates the phosphorylation of MKK7 by MAPKKKs on serine and threonine in a S-K-A-K-T motif in the

catalytic Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...

domain (kinase domain).

Signaling and regulation

MKK7 play an important part in the stress-activated protein kinase/c-Jun N-terminal kinase (SAP/JNK) signaling pathway. In collaboration with another mitogen-activated protein kinase kinase

MKK4 Dual-specificity mitogen-activated protein kinase kinase 4 is an enzyme that in humans is encoded by the ''MAP2K4'' gene. ''MAP2K4'' encodes a dual-specificity kinase that belongs to the Ser/Thr protein kinase family. MAP2K4 phosphorylates M ...

, MKK7 work as crucial transducers upstream of JNK signaling. Through joint efforts the two MKKs phosphorylate different JNK

s. As a result, MKK7 has a great impact on numerous physiological processes such as proliferation and differentiation, as well as pathological processes such as

apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

and

tumorigenesis Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abn ...

. MKK7 are activated as a result of cellular stresses. They are activated by a number of MKKKs through

at a S-K-A-K-T motif located in the MKK7s kinase domain. The MKKKs relate to MKK7 through its DVD site at the C-terminus and phosphorylate MKK7 at

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − ...

and

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form when dissolved in water), a carboxyl group (which is in the deprotonated −COO− ...

residues. Once activated, MKK4 and MKK7 directly phosphorylate specific

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...

and

residues located in the conserved T-P-Y motif of the activation loop of the JNK protein. Although MKK7 act through dual specificity it tends to phosphorylate

on JNK protein, leaving MKK4 to phosphorylate tyrosine. Phosphorylated and activated JNKs activate substrates like

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription (genetics), transcription of genetics, genetic information from DNA to messenger RNA, by binding t ...

s or pro-apoptotic protein. MKK7 and MKK4 seem to be regulating the expression of each other, thereby affecting the JNK signaling. The mono-phosphorylation of JNK on a threonine residue is adequate for the increase in JNK activity, which argues that MKK7 is an important constituent for JNK activity, while the additional phosphorylation of the tyrosine residue by MKK4 provide for a more favorable activation. Overall, MAP2K7 contains multiple amino acid sites that are phosphorylated and ubiquitinated.

Scaffold proteins

In addition to the direct interactions between JNK, MKK7 and other upstream protein kinases, various

scaffold protein In biology, scaffold proteins are crucial regulators of many key signalling pathways. Although scaffolds are not strictly defined in function, they are known to interact and/or bind with multiple members of a signalling pathway, tethering them in ...

s function to ensure specificity between the components of the MAPK signaling cascade. Different JNK isoforms, MAPK, and MAPKKs (e.g., MKK7 or MKK4) bind specifically to the scaffold proteins. Several mammalian scaffold proteins have been identified. These include the JNK-interacting protein (JIP) 1 and its closerly-related homolog, JIP2 or the (completely unrelated) JIP3 and JIP4 proteins. Nevertheless, JIP1/2 and JIP3/4 were shown to be capable of direct interaction with each other. Plenty of Src-homology-3 (POSH) has also been shown to be a partner of JIP1/2. All these JNK pathway regulators assemble transport complexes, tied to kinesin-dependent vesicular transport. In this context, JIP1/2 act as cargo adaptors, binding to a motor protein and a cargo protein simultaneously. In addition to their "normal" cargoes (C-termini of transmembrane proteins), they also transport MAP2K and MAP3K enzymes, namely MKK7, DLK and MLK3. Kinases bound to the JIP1/2 scaffold are generally sequestrated and thought to be inactive. Since the cargo-linkage mechanism of this complex is believed to be phosphporylation-dependent, phosphorylation by JNK kinase can release its own upstream activators from the scaffold, thus driving a strong local positive feedback loop.

Interactions

MAP2K7 has been shown to interact with: * CNKSR1 *

MAPK8IP3 C-jun-amino-terminal kinase-interacting protein 3 is an enzyme that in humans is encoded by the ''MAPK8IP3'' gene. The protein encoded by this gene shares similarity with the product of Drosophila syd gene, required for the functional interaction ...

MAPK8IP1 C-jun-amino-terminal kinase-interacting protein 1 is an enzyme that in humans is encoded by the ''MAPK8IP1'' gene. The protein encoded by this gene is a regulator of the pancreatic beta-cell function. It is highly similar to JIP-1, a mouse prote ...

GADD45B Growth arrest and DNA-damage-inducible, beta, also known as GADD45B, is a protein which in humans is encoded by the ''GADD45B'' gene. Function This gene is a member of a group of genes whose transcript levels are increased following stressful ...

* MAP3K12 * MAP3K2 *

MAPK8IP2 C-jun-amino-terminal kinase-interacting protein 2 is a protein or the name of the gene that encodes it. The gene is also known as Islet-Brain-2 (IB2). This protein is highly expressed in the brain and is almost always deleted in Phelan-McDermid ...

* DUSP19

Biological relevance

MKK7 is involved in the development of epithelial tissues such as skin and lungs, and also the developing teeth, during early embryogenesis in mice. Experiments also indicate that MKK7 in addition to MKK4 are required for mammalian body plan organization during

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

. MKK7 has also been suggested to function as a putative Metastase Suppressor Gene (MSG) by possibly promoting tumor dormancy at the

metastatic Metastasis is a pathogenic agent's spreading from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, ...

site. In small mammals, stress like pressure overload can cause cardiac hypertrophy and failure if MKK7 is knocked out. Conditional deletion of ''Map2k7'' in neural stem cells and postmitotic neurons identified a role for MKK7 in axonal elongation. Neuron-specific deletion of ''Map2k7'' showed a role for MKK7 in age-dependent motor dysfunction. Genetic variations in ''MAP2K7'' have been associated with schizophrenia in humans.