ELK1

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ELK1
Gene ontology
Molecular function
Cellular component
Biological process
Sources:Amigo / QuickGO
Ensembl
UniProt
RefSeq (mRNA)

NM_005229
NM_001114123
NM_001257168

NM_007922

RefSeq (protein)

NP_001107595
NP_001244097
NP_005220

NP_031948

Location (UCSC)Chr X: 47.64 – 47.65 MbChr X: 20.8 – 20.82 Mb
PubMed search[3][4]
Wikidata
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ETS Like-1 protein Elk-1 is a

.

Structure

Figure 1

As depicted in Figure 1, the Elk1 protein is composed of several domains. Localized in the N-terminal region, the A domain is required for the binding of Elk1 to DNA. This region also contains a

extracellular signal-regulated kinase (Erk), a type of MAPK, with Elk1.[6]

Expression

Given its role as a transcription factor, Elk1 is expressed in the nuclei of non-neuronal cells. The protein is present in the cytoplasm as well as in the nucleus of mature neurons.[6] In post-mitotic neurons, a variant of Elk1, sElk1, is expressed solely in the nucleus because it lacks the NES site present in the full-length protein.[7] Moreover, while Elk1 is broadly expressed, actual levels vary among tissues. The rat brain, for example, is extremely rich in Elk1, but the protein is exclusively expressed in neurons.[8]

Splice variants

Aside from the full-length protein, the Elk1 gene can yield two shortened versions of Elk1: ∆Elk1 and sElk1. Alternative splicing produces ∆Elk1. This variant lacks part of the DNA-binding domain that allows interaction with SRF.[9] On the other hand, sElk1 has an intact region that binds to SRF, but it lacks the first 54 amino acids that contain the NES. Found only in neurons, sElk1 is created by employing an internal translation start site.[10] Both ∆Elk1 and sElk1, truncated versions of full-length protein, are capable of binding to DNA and inducing various cellular signaling. In fact, sElk1 counteracts Elk1 in neuronal differentiation and the regulation of nerve growth factor/ERK signaling.[8]

Signaling

Figure 2

The downstream target of Elk1 is the serum response element (SRE) of the

growth factors to their corresponding receptors. However, Raf activation does not exclusively depend on Ras. Protein kinase C, which is activated by phorbol esters, can fulfill the same function as Ras.[22] MEK kinase (MEKK) can also activate MEKs, which then activate MAPKs, making Raf unnecessary at times.[23] Various signal transduction pathways, therefore, funnel through MEKs and MAPKs and lead to the activation of Elk1. After stimulation of Elk1, SRF, which allows Elk1 to bind to the c-fos promoter, must be recruited. The binding of Elk1 to SRF happens due to protein-protein interaction between the B domain of Elk1 and SRF and the protein-DNA interaction via the A domain.[6]

The aforementioned proteins are like recipes for a certain signaling output. If one of these ingredients, such as SRF, is missing, then a different output occurs. In this case, lack of SRF leads to Elk1's activation of another gene.[16] Elk1 can, thus, independently interact with an ETS binding site, as in the case of the lck proto-oncogene in Figure 2.[16] Moreover, the spacing and relative orientation of the Elk1 binding site to the SRE is rather flexible,[24] suggesting that the SRE-regulated early genes other than c-fos could be targets of Elk1. egr-1 is an example of an Elk1 target that depends on SRE interaction.[16] Ultimately, phosphorylation of Elk1 can result in the production of many proteins, depending on the other factors involved and their specific interactions with each other.

When studying signaling pathways, mutations can further highlight the importance of each component used to activate the downstream target. For instance, disruption of the C-terminal domain of Elk1 that MAPK phosphorylates triggers inhibition of c-fos activation.[16] Similarly, dysfunctional SRF, which normally tethers Elk1 to the SRE, leads to Fos not being transcribed.[21] At the same time, without Elk1, SRF cannot induce c-fos transcription after MAPK stimulation.[16] For these reasons, Elk1 represents an essential link between signal transduction pathways and the initiation of gene transcription.

Clinical significance

Long-term memory

Formation of

histones, increased interaction with SRF, and recruitment of the basal transcriptional machinery, all of which do not require direct binding of Elk1 to DNA.[6]

Drug addiction

Elk1 activation plays a central role in

opioids, one of the world's oldest known drugs. Altogether, these results highlight that Elk1 is an important component of drug addiction.[6]

Pathophysiology

Buildup of

amyloid precursor protein (APP). APP makes variants of Aβ (Aβ42/43 polypeptide). Moreover, PS1 is genetically associated with most early-onset cases of familial Alzheimer's disease. These data emphasize the intriguing link between Aβ, Elk1, and PS1.[6]

Another condition associated with Elk1 is Down syndrome. Fetal and aged mice with this pathophysiological condition have shown a decrease in the activity of calcineurin, the major phosphatase for Elk1. These mice also have age-dependent changes in ERK activation. Moreover, expression of SUMO3, which represses Elk1 activity, increases in the adult Down syndrome patient. Therefore, Down syndrome is correlated with changes in ERK, calcineurin, and SUMO pathways, all of which act antagonistically on Elk1 activity.[6]

Elk1 also interacts with BRCA1 splice variants, namely BRCA1a and BRCA1b. This interaction enhances BRCA1-mediated growth suppression in breast cancer cells. Elk1 may be a downstream target of BRCA1 in its growth control pathway. Recent literature reveals that c-fos promoter activity is inhibited, while overexpression of BRCA1a/1b reduces MEK-induced activation of the SRE. These results show that one mechanism of growth and tumor suppression by BRCA1a/1b proteins acts through repression of the expression of Elk1 downstream target genes like Fos.[25]

Depression has been linked with Elk1. Decreased Erk-mediated Elk1 phosphorylation is observed in the hippocampus and prefrontal cortex of post-mortem brains of suicidal individuals. Imbalanced Erk signaling is correlated with depression and suicidal behavior. Future research will reveal the exact role of Elk1 in the pathophysiology of depression.[6]

References

  1. ^ a b c GRCh38: Ensembl release 89: ENSG00000126767 - Ensembl, May 2017
  2. ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000009406 - Ensembl, May 2017
  3. ^ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. PMID 2539641
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External links

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