Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

MAP Kinase-Activated Protein Kinase 5 (MK5)

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_321

Synonyms

Historical Background

Murine and human MK5 cDNAs were initially isolated in 1998 in two independent screens for proteins with sequence homology to MK2 (New et al. 1998; Ni et al. 1998). The novel 54-kDa kinase ubiquitously expressed in all tissues displayed 45% amino acid identity to MK2. Both groups showed that MK5 could be phosphorylated and activated in vitro by the p38 MAP kinase, as detected by 32P incorporation into a substrate peptide (KKRPQRATSNVFS) or Hsp25 (HSPB1). New et al. named this kinase as p38-regulated and activated kinase or PRAK to emphasize its integration into the p38 pathway. More recently, MK5 has also been shown to interact with the atypical MAP kinases, ERK3 (MAPK6) and ERK4 (MAPK4), and these kinases are also involved in the phosphorylation and activation of MK5 (Schumacher et al. 2004; Seternes et al. 2004; Aberg et al. 2006; Kant et al. 2006). An MK5 gene does not appear to be present in either C. elegans or Drosophila, but orthologs are found in most vertebrates.

Structure, Activation, and Expression

MK5 belongs to the family of calcium/calmodulin-dependent protein kinases. The kinase domain of MK5 is most closely related to MK2 (52% amino acid identity) and MK3 (50% identity), but MK5 is more distantly related to these kinases than they are to each other (77%). It is a ubiquitously expressed kinase with expression detectable in most of the tissues and cell lines analyzed so far. The human MK5 gene undergoes alternative splicing, resulting in two mRNAs encoding isoforms differing in just two additional amino acids within the C-terminal extension of MK5, with the longer transcript (variant 2) coding for a 473 aa protein. In mice, five MK5 isoforms have been detected, representing combinations of the two amino acid changes found in humans with deletions of the N-terminal portion of the MK5 catalytic domain as well as a truncated variant due to frame shift (Dingar et al. 2010). The functional significance of these different splice variants is yet unknown. Similar to the other MKs, MK5 contains a conserved LXTP motif in the T-loop harboring the activating phospho-acceptor threonine, where X is threonine for MK2, glutamine for MK3, and methionine for MK5. The C-terminal region of MK5 contains a functional nuclear export signal (NES) and a nuclear localization signal (NLS). The NLS overlaps with a putative MAPK docking site (D motif) shown to mediate interaction with p38 (Seternes et al. 2002). In contrast to MK2 and MK3, the NES and NLS in MK5 are located in close proximity to each other. In addition, MK5 possesses a 100 amino acid extension C-terminal to the NLS, which is absent in other MKs. This extension is both necessary and sufficient for the interaction of MK5 with phosphorylated MAPKs ERK3/ERK4 and enables alternative modes of MK5 activation. This unique capability of MK5 to be activated by the atypical MAPKs in addition to p38, separates it from MK2/3 and makes it a converging point for two distinct signaling pathways (Fig. 1).
MAP Kinase-Activated Protein Kinase 5 (MK5), Fig. 1

Schematic structure of MK5. The activating T-loop phosphor-site T182 & the PKA-target site residue -S115 are indicated in the kinase domain. The nuclear export sequence (NES), the D-domain/nuclear localization signal (NLS), and the ERK3/4 binding domain are depicted

T182 located in the activation loop LXTP motif of MK5 is the most well characterized activating phosphorylation site on MK5. While MK2/3 requires phosphorylation at additional regulatory MAPK sites at the C-terminus for full activation, there are no such characterized sites on MK5. A recent large-scale screen identified strong CXCL12-induced phosphorylation of MK5 at T368, but neither the significance of this phosphorylation on MK5 activity nor the upstream kinase involved has been analyzed so far (Yi et al. 2014). Interestingly K364-acetylation of MK5 was shown to enhance its activity (Zheng et al. 2013). p38 can directly phosphorylate T182 and activate MK5 akin to MK2 (New et al. 1998; Seternes et al. 2002) at least in vitro and upon overexpression. Additionally, overexpressed p38 can bind MK5 leading to cytoplasmic relocalization of the resulting complex. However, endogenous MK5 is not significantly activated by classical p38 stimuli such as arsenite and sorbitol (Shi et al. 2003). Moreover, the phenotype of MK5-deficient mice does not resemble one of MK2/3-deficient animals, displaying a normal profile of cytokine production and no increased resistance to LPS challenge (Shi et al. 2003). These observations argue against the physiological relevance of the p38-MK5 signaling axis.

MK5 was shown to interact with atypical MAPKs, ERK3 and ERK4. These kinases form a tight complex with MK5 resulting in mutual stabilization and phosphorylation. Moreover, the coexpression of ERK3/4 not only leads to phosphorylation and activation of MK5 but also cause the translocation of ERK3–MK5 from the nucleus to the cytoplasm (Schumacher et al. 2004; Seternes et al. 2004). However, the physiological conditions, leading to the activation of MK5 by ERK3 and ERK4 remain unknown. Recent studies have identified p21-activated kinases (PAKs) as activators of ERK3/4, suggesting that the cytoskeletal small-GTPases which regulate PAKs could be involved in the activation of ERK3/4 signaling (De la Mota-Peynado et al. 2011; Deleris et al. 2011). In addition to the atypical MAPKs and p38, Protein kinase A (PKA) is another kinase which has been shown to influence the cellular distribution of MK5 (Gerits et al. 2007a). Treatment of cells with forskolin or overexpression of a nuclear-targeted PKAc-α (catalytic subunit) was shown to induce the transient redistribution of MK5 from nucleus to cytoplasm in rat PC12 cells. Interestingly, PKA modulates MK5 functions independent of T182 by phosphorylating it at S115 (Kostenko et al. 2011).

Substrates

The optimal phosphorylation site motif for MK5 is very similar to MK2/3 and has been defined as (L,M,F,Y,W)-X-R-(Q,M,S)-X-(pS,pT)-X (Ronkina et al. 2015). In vitro, MK5 is able to phosphorylate HSPB1, glycogen synthase, tyrosine hydroxylase (preferentially at S19), and myosin heavy chain (New et al. 1998; Ni et al. 1998; Toska et al. 2002; Gaestel 2006) at the same sites as MK2. Additional in vitro substrates of MK5 include SEPT8 (Shiryaev et al. 2012), BORG (binder of Rho-GTPase) proteins (CDC42EP3 & CDC42EP5), KAL7 (Brand et al. 2012), DNAJB1 (Kostenko et al. 2014), FAK (Dwyer and Gelman 2014), and DJ-1 (Tang et al. 2014).

While cAMP/PKA-induced HSPB1- phosphorylation and F-actin remodeling were shown to be MK5-dependent (Kostenko et al. 2011), stress-dependent phosphorylation of HSPB1 is not impaired in the MK5-KO mice (Shi et al. 2003). A stimulus-specific role for MK5 in HSPB1-phosphorylation is yet to be verified in knockout models. MK5 phosphorylates its interacting partners, atypical MAPKs ERK3 (Schumacher et al. 2004; Seternes et al. 2004) as well as homologous ERK4 (Aberg et al. 2006; Kant et al. 2006). RHEB phosphorylation at S130 was shown to be mediated by MK5 (Zheng et al. 2011) and MK5 was also reported to be the kinase responsible for S37 phosphorylation of p53, downstream to oncogenic Ras-p38 signaling (Sun et al. 2007). In addition, independent studies have shown a role for MK5 in the phosphorylation of the fork-head family transcription factors FOXO1 and FOXO3a at a conserved residue (S215) in the DNA-binding domain (Kress et al. 2011; Chow et al. 2013).

Physiological Functions

The exact biological function of MK5 is largely unknown. Originally, due to the high structural similarity and substrate consensus, MK5 was expected to be functionally similar to MK2, which is involved in stress response and inflammation. Generation of the MK5-deficient mice challenged this assumption (Shi et al. 2003). Indeed, MK5-deficient mice do not display any of the phenotypic changes characteristic of MK2-deficient animals. Disruption of the MK5 gene in mice on mixed genetic background does not manifest any phenotypical features. MK5 knockout animals backcrossed to C57Bl/6 genetic background resulted in lethality at E11.5 with incomplete penetrance, indicating a role for MK5 in embryogenesis (Schumacher et al. 2004). In studies exploring the Ras-induced senescence, using a second knockout model, MK5 was proposed to be a tumor suppressor with active role in senescence (Sun et al. 2007). In the same study, the authors demonstrated that MK5 phosphorylates p53 at S37, a residue located in the transactivation domain. However, this residue does not lie within the consensus phosphorylation motif for MK5. In addition, a recent report comparing the two different knockout models of MK5 present strong evidence against this proposed role for MK5 as a tumor suppressor in regulating senescence (Ronkina et al. 2015). However, MK5 acts as a tumor suppressor in colon cancer by inhibiting the translation of tumor promoting c-MYC. In this system, MK5 was shown to phosphorylate and activate FOXO3a inducing the expression of microRNA miR-34b/c, which in turn suppresses c-MYC translation (Kress et al. 2011). In addition, there is increasing evidence for the involvement of ERK3 in tumorigenesis and metastasis which makes MK5 a key player in these processes as a modulator of stability, activation, and subcellular localization of ERK3.

Overexpression of MK5 in HeLa cells leads to an increase in both F-actin content and cell migration, an effect which was shown to be counteracted by 14–3-3 epsilon (Tak et al. 2007). MK5 interaction with 14–3-3 epsilon was shown to decrease its kinase activity towards HSPB1. In a contrasting report, MK5-overexpression was shown to suppress cell migration and mutual phosphorylation by Focal Adhesion Kinase (FAK) and MK5 was proposed to regulate FAK-activation and cell migration (Dwyer and Gelman 2014). Another study utilized siRNA-mediated knockdown of MK5 in PC12-cells to show that MK5 is necessary for the forskolin-induced transient increase in F-actin levels (Gerits et al. 2007a). How MK5 mediates cytoskeletal rearrangements and cell migration is still unclear, and further studies are needed to address this.

Finally, a possible link between MK5 and neurological/cognitive function was proposed based on behavioral analyses of a transgenic mouse that expresses a constitutively active mutant of MK5. This study revealed complex sex-specific changes in both anxiety-related traits and locomotor activity in MK5 expressing mice relative to WT controls (Gerits et al. 2007b). Interestingly, the analysis of hippocampal sections from the MK5 knockout mice revealed significantly lower numbers of dendritic spines suggesting a role for MK5 in neuronal morphogenesis (Brand et al. 2012). Yeast two-hybrid screens and further analysis led to the establishment of a signaling complex consisting of ERK3, MK5, and SEPT7 as modulators of dendritic branching and spine formation (Brand et al. 2012). In addition, septin-interacting BORG family proteins and KAL7, a Rho-GEF with role in dendritic spine morphogenesis, were identified as in vitro substrates of MK5. Thus, the regulation of neuronal morphogenesis seems to be the only well-established physiological function of the ERK3/MK5 signaling module (Fig. 2).
MAP Kinase-Activated Protein Kinase 5 (MK5), Fig. 2

Physiological functions of MK5 signaling. ERK3 is an intrinsically unstable protein and can be stabilized by differentiation or in development. Mechanism of ERK4 regulation is not revealed until now, but PAKs have been identified as activating kinases for ERK3/4 recently. ERK3 and ERK4 bind to MK5 and mutually phosphorylate and activate each other. p38 is able to phosphorylate MK5 at T182. However, in vivo evidence for the dependence of p38-activity for MK5 phosphorylation is limited as compared to that for MK2/3 and this fact is indicated with dotted lines between p38 and MK5. PKA-mediated phosphorylation of MK5 at S115 is also shown. The major interaction partners and substrates of MK5 and their contributions to three major physiological outputs of MK5 signaling (1) neuronal morphogenesis, (2) tumor suppression, and (3) actin remodeling and migration are represented. The MK5 mediated p53 phosphorylation is denoted with a dotted line due to the recent evidences against a direct role for MK5 in this process. Involvement of MK5 in the regulation of actin reorganization is described and could be mediated by MK5-dependent phosphorylation of F-actin capping proteins, like the small heat shock protein HSPB1. Even though MK2/3 seems to be the major HSPB1 kinase, MK5 could mediate HSPB1 phosphorylation in response to specific PKA-activating stimuli

Summary

MK5 is a MAPK-activated kinase integrated into the p38 and ERK3/ERK4 signaling pathways. p38 was initially described as the major MAPK activating MK5, resulting in the name PRAK, which is an acronym for p38 regulated and activated kinase. However, p38 activating stimuli do not usually cause MK5 phosphorylation and activation. Hence, the significance of p38 in MK5 signaling is restricted and needs to be directly addressed in future studies. In contrast, the involvement of ERK3 and ERK4 in MK5 functions is clearly documented in in vivo studies and supported by strong interaction between these proteins, mutual stabilization, and similar pattern of expression in embryogenesis. However, the ERK3/4- MK5 pathway still remains an “orphan” signaling module due to lack of information regarding physiological stimuli as well as regulatory mechanisms. While MK5 seems to have some impact on tumorigenesis, actin remodeling, and cell migration, ERK3-MK5-SEPT7-mediated neuronal morphogenesis seems to be the sole established physiological function of this cascade. Future studies in MK5 knockout animals and dissection of signaling events using genetic approaches involving the p38, ERK3/4, and MK2/3 knockout models and MK5 inhibitors are necessary to further characterize and understand MK5 signaling.

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute of Cell Biochemistry, Hannover Medical School (MHH)HannoverGermany
  2. 2.Institute of Physiological Chemistry, Hannover Medical School (MHH)HannoverGermany