Phosphatase Subfamily ILKAP
Phosphatase Classification: Fold PPM (PP2C): Superfamily PPM (PP2C): Family PPM (PP2C): Subfamily ILKAP
The ILKAP subfamily emerged in holozoa and was lost in arthropods. In contrast, its physical interacting partners or substrates, ILK and RSK are present in arthropods.
The ILKAP subfamlily has a phosphatase domain and an N-terminal region binding to importin which localizes ILKAP to nuclei .
The ILKAP gene is widely expressed across different tissues as shown by Northern blot analysis in rat  and RNA-seq data from GTEx in human.
Human ILKAP is localized to nuclei by interacting with importin through a conserved region from 71-86, particularly Lys-78 and Arg-79 are critical . It is also localized to cytoplasm and focal adhension, as implied by its interaction with ILK .
ILKAP interacts with ILK and indirectly modules GSK3β Ser-9 phosphorylation
Human ILKAP associated with integrin-linked kinase (ILK) in a manner independent of their catalytic activies . Catalytically active but not mutant ILKAP (H154D) strongly inhibited ILK's substrate, glycogen synthase kinase 3 β (GSK3β), at Ser-9, but did not affect the phosphorylation of another substrate, protein kinase B (PKB/AKT) at Ser-473. However, ILKAP may not directly dephosphorylate Ser-9 on GSK3β [3, 4]. A model was proposed in  (figure 7), in which ILKAP, localized in the cytoplasm, inhibits ILK, promotes GSK3β-dependent Thr-286 phosphorylation, nuclear export and proteolysis of cyclin D1.
ILK and ILKAP emerged in holozoa; however, ILK is retained in arthropods, while ILKAP is lost (see ILK subfamily in KinBase.
ILKAP dephosphorylates ERK and RSK2
ILKAP binds to the N-terminal kinase domain of p90 ribosomal S6 kinase 2 (RSK2) via its N-terminal region (1071)  in the nculeus . It partially dephosphorylates RSK2 at four sites phosphorylated by ERK, Ser-227 in the activation loop of te N-terminal kinase domain, Ser-369 in the linker, Ser-386 in the hydrophobic motif and Thr-577 in the C-terminal kinase domain (RSKs have two functional kinase domains) . The dephosphorylation reduced RSK2 activity and down-regulated RSK2's substrate cyclin D1 .
ILKAP also associated with other kinases of AGC group, such as other RSKs, MSK1 and MSK2, p70S6K, and PDK1. ILKAP also dephosphorylates ERK at Thr-315 and Thr-333 in the catalytic domain .
RSK emerged in holozoa; but in contrast with ILKAP, it is present in arthropods (see RSKp90 subfamily in KinBase).
ILKAP binds to Palladin
ILKAP binds to Palladin (gene symbol: PALLD), an actin-associated scaffold protein involved in the formation of actin-associated protein complexes, localized in stress fibers, focal adhensions, growth cones and etc. The interaction is mediated by Palladin's last three Ig domains (710-1106) and ILKAP's phosphatase domain (108-392) . Notice: do not confuse Palladin with Paladin, which is a pseudophosphatase.
Palladin emerged in chordates and duplicated in vertebrates, which resulted in two members in human genome, Palladin and Myopalladin (internal database).
- Zhou W, Cao H, Yang X, Cong K, Wang W, Chen T, Yin H, Wu Z, Cai X, Liu T, and Xiao J. Characterization of nuclear localization signal in the N terminus of integrin-linked kinase-associated phosphatase (ILKAP) and its essential role in the down-regulation of RSK2 protein signaling. J Biol Chem. 2013 Mar 1;288(9):6259-71. DOI:10.1074/jbc.M112.432195 |
- Tong Y, Quirion R, and Shen SH. Cloning and characterization of a novel mammalian PP2C isozyme. J Biol Chem. 1998 Dec 25;273(52):35282-90. DOI:10.1074/jbc.273.52.35282 |
- Leung-Hagesteijn C, Mahendra A, Naruszewicz I, and Hannigan GE. Modulation of integrin signal transduction by ILKAP, a protein phosphatase 2C associating with the integrin-linked kinase, ILK1. EMBO J. 2001 May 1;20(9):2160-70. DOI:10.1093/emboj/20.9.2160 |
- Kumar AS, Naruszewicz I, Wang P, Leung-Hagesteijn C, and Hannigan GE. ILKAP regulates ILK signaling and inhibits anchorage-independent growth. Oncogene. 2004 Apr 22;23(19):3454-61. DOI:10.1038/sj.onc.1207473 |
- Doehn U, Gammeltoft S, Shen SH, and Jensen CJ. p90 ribosomal S6 kinase 2 is associated with and dephosphorylated by protein phosphatase 2Cdelta. Biochem J. 2004 Sep 1;382(Pt 2):425-31. DOI:10.1042/BJ20040948 |
- Zhou W, Cui S, Han S, Cheng B, Zheng Y, and Zhang Y. Palladin is a novel binding partner of ILKAP in eukaryotic cells. Biochem Biophys Res Commun. 2011 Aug 12;411(4):768-73. DOI:10.1016/j.bbrc.2011.07.022 |