The ILK/PINCH/parvin complex: the kinase is dead, long live the pseudokinase!

Abstract

Dynamic interactions of cells with their environment regulate multiple aspects of tissue morphogenesis and function. Integrins are the major class of cell surface receptors that recognize and bind extracellular matrix proteins, resulting in the engagement and organization of the cytoskeleton as well as activation of signalling pathways to regulate cell behaviour and morphogenetic processes. The ternary complex of integrin-linked kinase (ILK), PINCH, and parvin (IPP complex), which was identified more than a decade ago, interacts with the cytoplasmic tail of beta integrins and couples them to the actin cytoskeleton. In addition, ILK has been shown to act as a serine/threonine kinase and to directly activate several signalling pathways downstream of integrins. However, the kinase activity of ILK and the precise functions of the IPP complex have remained elusive and controversial. This review focuses on the recent advances made towards understanding the specialized roles this complex and its individual components have acquired during evolution.

Figure 1

The kinase homology domain of ILK lacks several critical residues present in eukaryotic serine/threonine kinases. Alignment of highly conserved amino acids within the 12 subdomains of eukaryotic serine/threonine kinases (Hanks et al, 1988) with Drosophila and Mus musculus ILK. Conserved amino acids are marked in bold, and their counterparts in the ILK sequences are shown in red. Conserved amino acids that are not present in the ILK sequences are highlighted with blue boxes. Most importantly, ILK lacks the catalytic base in subdomain VIb. Substitution of the invariant lysine in subdomain II (ATP binding) to either an alanine or methionine (K220A/M in M. musculus ILK; marked with ^*) or of the invariant glutamate in subdomain VIII (substrate recognition) to a lysine (E359K in M. musculus ILK; marked with ^**) have been shown to render ILK catalytically inactive.

Figure 2

Model of how the ILK/α-parvin complex regulates tissue morphogenesis by suppressing cell contractility. (A–C) Examples of morphogenetic events that require tight regulation of cell contractility for promoting directional migration and cellular organization. (A) Kidney development is initiated by the outgrowth of the ureteric bud (UB) from the Wolffian duct (WD) to invade the surrounding metanephric mesenchyme (MM) resulting in nephron formation and formation of the collecting duct system. BM, basement membrane. (B) Vascular smooth muscle cells (vSMC) and pericytes (PC) are recruited by the endothelial cells (EC) to surround large arteries and capillaries, respectively. The recruited vSMCs and PCs subsequently spread around the vessels to stabilize the endothelial tubes, to guide vascular remodelling, and to regulate vessel tone. (C) Schwann cells extend cellular processes and wrap axons of neurons to generate a myelin sheet that promotes neuronal survival and enhances the conduction velocity of nerve impulses. (D) Two molecularly distinct IPP complexes associate with integrins at cell-matrix adhesions. The ILK/PINCH/α-parvin complex functions as a mechanosensor to downregulate RhoA signalling downstream of cell adhesion, possibly by recruiting an unidentified negative regulator of RhoA (illustrated with a boxed question mark). Suppression of RhoA activity leads to decreased activation of the downstream target ROCK. ROCK regulates contractility by inactivating the myosin light chain phosphatase (MYPT), which dephosphorylates and inactivates myosin light chain (MLC). ROCK has also been shown to directly phosphorylate and activate MLC. Activated MLC promotes actomyosin contractility. Spatiotemporal regulation of RhoA activity and actomyosin contractility is essential to promote actin remodelling, cell spreading, and directed cell migration. Enhanced RhoA activity, which occurs in the absence of ILK/α-parvin, leads to actomyosin hypercontractility resulting in enhanced actin stress fibre formation, decreased cell spreading, and loss of directional migration. The specific functions of the ILK/PINCH/β-parvin complex are not clear, but are likely to involve remodelling of the actin cytoskeleton downstream of integrin adhesion.