The secret life of kinases: functions beyond catalysis

Abstract

Protein phosphorylation participates in the regulation of all fundamental biological processes, and protein kinases have been intensively studied. However, while the focus was on catalytic activities, accumulating evidence suggests that non-catalytic properties of protein kinases are essential, and in some cases even sufficient for their functions. These non-catalytic functions include the scaffolding of protein complexes, the competition for protein interactions, allosteric effects on other enzymes, subcellular targeting, and DNA binding. This rich repertoire often is used to coordinate phosphorylation events and enhance the specificity of substrate phosphorylation, but also can adopt functions that do not rely on kinase activity. Here, we discuss such kinase independent functions of protein and lipid kinases focussing on kinases that play a role in the regulation of cell proliferation, differentiation, apoptosis, and motility.

Figure 1

Examples of catalytic-independent functions in yeast. (A) The dual role of PBS2 as scaffold and kinase in yeast osmo-sensing pathways. (B) Antagonistic functions of the kinase Kss1 in filamentation and invasion.

Figure 2

Kinase actitvity-independent functions of the EGFR/ERBBs. (A) EGFR prevents autophagic cell death by stabilizing the sodium/glucose cotransporter (SGLT1) thus maintaining the basal intracellular glucose level. (B) In glioblastomas, EGFR and EGFR vIII sequester the proapoptotic Bcl-2 family member PUMA in the cytoplasm leading to tumour drug resistance. (C) EGFR and ERBB4 regulate gene expression by direct interaction with transcription factors in the nucleus.

Figure 3

Kinase-independent functions of Raf kinases. (A) General structure of MAPK pathways. (B) Raf-1 controls cell migration and differentiation by inhibiting the Rho effector kinase ROK-α. (C) Raf-1 controls TNF- and Fas-mediated apoptosis by inhibiting apoptosis signal-regulating kinase-1 (ASK). (D) Raf-1 and A-Raf bind and inhibit the pro-apoptotic mammalian sterile 20-like kinase (MST2) thereby interfering with its dimerization, autophosphorylation, and activation.

Figure 4

Catalytic-independent functions of MAPKs. (A) Topoisomerase IIa, involved in winding and unwinding of DNA, is activated by ERK by a phosphorylation-independent process. (B) Activated ERK2 interacts with PolyADP-ribose polymerase 1 (PARP-1) and activates it independent of ERK kinase activity and DNA strand breaks. (C) ERK can repress INFγ induced genes by directly binding to a specific DNA sequence and displacing the CEBP-β transcription factor. (D) Activated ERK1 and ERK2 regulate the cell cycle entry by dislodging Rb from its interaction with lamin A. Rb is released to the nucleoplasm and is rapidly phosphorylated and inactivated, leading to activation of the transcription factor E2F and cell cycle entry. (E) p38 inhibits cell cycle progression by blocking Mirk/Dyrk1B transcriptional activity in proliferating cells.

Figure 5

Non-catalytic functions of FAK and PAK kinases in cell motility and survival. (A) FAK mediated translocation of paxillin to the cell membrane initiates activation of the JNK pathway and cell motility by recruiting the Rac/Cdc42 exchange factor PIX. (B) FAK and the related kinase Pyk2 can translocate to the nucleus where they bind p53 and Mdm2 to induce p53 ubiquitination and degradation, promoting cell proliferation and survival. (C) PAK1 regulates focal adhesion dynamics by promoting paxillin recruitment to focal adhesions. (D) Chemotactic cytokines via Gβ/γ proteins induce a positive feedback activation of Cdc42 by activating Rac, which in turn activates the Rac/Cdc42 exchange factor PIX. (E) PAK1 scaffolds the activation of Akt1 by PDK1 at the cell membrane and coordinates the selective phosphorylation of downstream Akt1 substrates.

Figure 6

PI3K signaling independent of catalytic activity. (A) PI3Kβ binding to DNA double strand breaks (DSBs) recruits DNA damage sensor and DNA repair enzymes. (B) PI3Kγ comprising of the p110γ and p87, catalytic and regulatory subunits, serves as scaffold for a Protein Kinase A (PKA) and phosphodiesterase 3B (PDE3B) containing protein complex that regulates β-adrenergic receptor internalization and muscle contractility in cardiomyocytes.

Figure 7

Catalytic-independent functions of PDK1. (A) PDK1 induces activation of the Ral small G protein by binding to and relieving its GEF, Ral-GDS from the autoinhibitory conformation. (B) PDK1 mediates NFB activation in T-cells by serving as a scaffold that brings CARMA1-BCL10-MALT1complex proximal to the PKCq-bound IKK complex, thus allowing ubiquitination of NEMO (IKKγ). This leads to activation of IKKs, phosphorylation and subsequent degradation of the NFκB inhibitor IκB and release of active NFκB into the nucleus.