ILK interactions.

Abstract

Integrin-linked kinase (ILK*) is a multidomain focal adhesion protein that is critically involved in adhesion of cells to the extracellular matrix (ECM) and signal transduction. The figure shows the interactions involving ILK and other related proteins and the downstream signals these elicit. ILK interacts with the cytoplasmic domains of β integrins (e.g. β1 integrin). Furthermore, it binds with high affinity to PINCH, a focal adhesion protein comprising five LIM domains. The ILK-PINCH binding is mediated by the N-terminal ankyrin (ANK) repeat domain of ILK and the second Zn finger located within the N-terminal-most LIM domain (LIM1) of PINCH. ILK also interacts with CH-ILKBP, an actin-binding focal adhesion protein containing two calponinhomology (CH) domains. Human CH-ILKBP is structurally closely related to rat actopaxin, a paxillin- and actin-binding protein. In addition, ILK can bind to the paxillin LD1 motif. The interaction between ILK and CH-ILKBP is mediated by the C-terminal domain of ILK and the CH2 domain of CH-ILKBP. Simultaneous interactions mediated by the two separate (the N- and C-terminal) domains of ILK result in the formation of a stable PINCH—ILK—CH-ILKBP ternary complex in cells. The formation of this complex facilitates the localization of PINCH, ILK and CH-ILKBP to cell-ECM contact sites, at which they provide a crucial physical connection between transmembrane receptors such as integrins and the actin filaments. This function of the PINCH—ILK—CH-ILKBP complex is probably conserved in organisms ranging from nematodes and insects to humans.FIG1*Abbreviations used: ANK, ankyrin; CH, calponin homology domain; CH-ILKBP, calponin-homology-domain-containing ILK-binding protein; ECM, extracellular matrix; FA, focal adhesion; GSK-3; glycogen synthase kinase 3; ILK, integrin-linked kinase; IRS-1, insulin receptor substrate 1; MAPK, mitogen-activated protein kinase; PAK, p21-activated kinase; PDGF, platelet-derived growth factor; PI3K, phosphoinositide 3-kinase; PtdIns(3,4,5)P3, phosphatidylinositol 3,4,5-trisphosphate; PH, pleckstrin homology domain; PKB, protein kinase B; RTK, receptor tyrosine kinase, SH, Src homology domain; WASP, Wiskott-Aldrich syndrome protein.ILK not only serves as an important molecular scaffold at the cell-ECM adhesion sites but also participates in signal transduction pathways that control cell survival, differentiation, proliferation and gene expression in mammalian cells. Important downstream targets of ILK signaling include PKB/Akt, GSK-3, β-catenin, p44/42 MAP kinases and the myosin light chain. Upstream events that regulate ILK activity include cell-ECM interactions and insulin stimulation, which are probably mediated by P13K. An ILK-associated serine/threonine phosphatase, termed ILKAP, has also been implicated in the regulation of GSK-3 phoshorylation. One of the potential convergence points between cell-adhesion- and growth-factor-mediated signal transduction pathways could be Nck-2, an adaptor protein that contains three N-terminal SH3 domains and one C-terminal SH2 domain. Nck-2 interacts with the LIM4 domain of PINCH through its third SH3 domain. In addition, it can associate with IRS-1 through its multiple SH3 domains and with ligand-activated growth factor receptors such as PDGF receptor β through its SH2 domain. Thus, the interactions mediated by multiple Nck-2 domains could regulate the formation of supramolecular complexes that contain components from both cell-adhesion- and growth-factor-signaling pathways and thereby facilitate communication between them (although the interactions mediated by Nck-2 have been demonstrated experimentally, they are depicted by red double arrows to indicate the hypothetical nature of the function of Nck-2 in such communication). Additional Nck-2-binding proteins include PAK, WASPs and DOCK 180, which are known to play important roles in Cdc42- and Racmediated actin cytoskeleton dynamics. Solid double arrows indicate proteinprotein interactions; dashed double arrows indicate putative intermolecular interactions; open arrows indicate phosphorylation reactions.