View source for Kinase Subfamily RIPK-A

__NOTOC__
[[kinase classification|Kinase Classification]]: [[Kinase_Group_TKL|Group TKL]]: [[Kinase_Family_RIPK|Family RIPK]]: Subfamily: RIPK-A

RIPK-A is a subfamily of kinases with ankyrin repeats that are involved in cell migration and neuronal behaviors.

====Evolution====
Two RIPK-A are found in vertebrates, RIPK4/ANKRD3 and ANKK1/SgK288. Invertebrate homologs may exist in other deuterostomes, but the rapid evolution of the gene sequences make it hard to be certain that they are of the same origin, and none have been experimentally studied. RIPK4 contains an insert between the two domains that is missing from ANKK1 and includes a cluster of serine phosphorylation sites ([https://www.phosphosite.org/proteinAction.action?id=573&showAllSites=true Phosphosite]), which constitute a conserved phosphodegron (DSAFSS) for the bTrCP ubiquitin ligase <cite>Tanghe</cite>. ANKK1 is very poorly phosphorylated.

====Domain Structure====
Both vertebrate members have an N-terminal kinase domain and multiple C-terminal ankyrin repeats, with an unannotated region in between. Sea urchins have possible orthologs that have N-terminal ankyrin repeats, and lancelets (invertebrate chordates) have RIPKs with either N-terminal or C-terminal repeats.

====Functions====
RIPK4 is involved in cell adhesion, EMT and migration, and linked to cancers, including ovarian and melanoma. RIPK4 interacts with several major signaling pathways. RIPK4 upregulates the Wnt pathway by binding and phosphorylation of Dishevelled (Dvl2) <cite>Huang</cite>. 

Overexpression and knockdown shows that RIPK4 activates NF-kB signaling in B cells <cite>Kim</cite> and melanoma cells <cite>Madej</cite>. Activation may be both dependent and independent of kinase activity <cite>Moran</cite> and in this system, RIPK4 may be activated by [[Kinase_Subfamily_MEKK2|MEKK2/3]] kinases. Kinase-dead mutants can block activation of NF-kB by phorbol esters.

RIPK4 repressed activation of the STAT3 transcription factor in a lung and hepatocellular models <cite>Kopparam, Li</cite>, where it acts as a tumor suppressor, though the mechanisms are not known. By contrast, it acts as a cancer driver through activating EMT in ovarian and other cancers, possibly through activation of IL-6 signaling <cite>Yi</cite>.

Human mutations in RIPK4 give rise to several disorders of epithelial development, including Bartsocas-Papas syndrome and CHAND, and RIPK4 mutant mice have limb and tail defects, epidermal and skin barrier malformations, and impaired keratinocyte differentiation. RIPK4 phosphorylates IRF6, a key transcription factor in keratinocytes, and which has a similar human disease phenotype.

RIPK4 was first found as a Y2H partner of Protein Kinase C isoforms PKCb and PKCd (Chen et al., 2001), and activation of PKC signaling by phorbol esters or expression of PKCe/PKCh/PKCd induced RIPK phosphorylation and activation and subsequent degradation <cite>Tanghe, Madej</cite>.


ANKK1 has been been little studied biochemically, but is genetically linked to many human behavioral functions (particularly addiction and reward-seeking) by genetics, especially the Taq1A SNP (rs1800497, Taq1A), which causes an E713K change at the end of the ankyrin repeats (K is found in most primates and most other mammals, with the exception of dogs and some of their relatives). ANKK1 is next to the dopamine receptor, DRD2, and many of these functions are also influenced by DRD2. A recent [https://www.biorxiv.org/content/10.1101/2022.08.12.503577v1.abstract preprint] shows that ANKK1 is co-expressed with DRD2 in striatal neurons and that the Taq1A SNP reduces DRD2 expression substantially and mouse knockdowns of ANKK1 have similar behavioral problems. ANKK1 mutation in zebrafish also causes a decreased in DRS2 expression <cite>Leggieri</cite>. Another coding SNP in the ankyrin repeats, rs2734849 (H490R, reported as R490H), is also associated with behavioral changes and decreases the expression of downstream NK-kB related genes in a cell line assay <cite>Huang</cite>. This H is also relatively poorly conserved in vertebrates. Another SNP, rs7118900, commonly in lineage disequilibrium with Taq1L, changes A239T, resulting in lower levels of ANKK1 protein in transfected cells <cite>Garrido</cite>.  Other SNPs in ANKK1 are also associated with behavioral changes <cite>Dick</cite>, though several are in linkage disequilibrium with Taq1A <cite>McAllister</cite>.

RIPK4 is most highly expressed in esophagus as well as skin and other barrier tissues [https://gtexportal.org/home/gene/RIPK4 GTEx]. It is mostly weakly expressed in the brain. ANKKs is more highly expressed in brain [https://gtexportal.org/home/gene/ANKK1 GTEx], supporting its neuronal role, but is also highly expressed in skin, suggesting an overlapping function with that of RIPK4.

====References====
<biblio>
#Huang pmid=23371553
#Kim pmid=18025152
#Kopparam pmid=28574510
#Li pmid=34222329
#Yi pmid=33855091
#Tanghe pmid=29435596
#Madej pmid=34768934
#Moran pmid=12676934
#Dick pmid=17850642
#McAllister pmid=18698520
#Leggieri pmid=35210987
#Huang pmid=18354387
#Garrido pmid=20845092
</biblio>