Differential tyrosine phosphorylation controls the function of CNK1 as a molecular switch in signal transduction.

Scaffold proteins are multidomain proteins without enzymatic function that play a central role in coordinating signaling processes. The scaffold protein CNK1 interacts with pathway-specific signaling proteins and thereby regulates these respective pathways. Here, we revealed tyrosine phosphorylation as a critical regulation mechanism to control the function of CNK1. We identified Tyr 26 as a PDGF-induced and, additionally, Tyr 519 and Tyr 665 as SRC-induced tyrosine phosphorylation sites. Phosphomimetic mutants indicate that phosphorylation of Tyr 519 recruits CNK1 to the nucleus and additional phosphorylation of Tyr 26 enables CNK1 to promote SRE-dependent gene expression. Contrary, mutants preventing tyrosine phosphorylation promote matrix metalloproteinase MMP14 promoter activity. CNK1-driven cell proliferation partially depends on its tyrosine phosphorylation. Upon PDGF stimulation, CNK1 is recruited to the plasma membrane mediated by SRC. Knock down of CNK1 prevents PDGF-induced SRE-dependent gene expression, MMP14 promoter activity and cell proliferation. Thus, tyrosine phosphorylation is an important mechanism to control the subcellular localization of CNK1 and its distinct biological functions.

EphrinB1 interacts with CNK1 and promotes cell migration through c-Jun N-terminal kinase (JNK) activation.

The Eph receptors and their membrane-bound ligands, ephrins, play important roles in various biological processes such as cell adhesion and movement. The transmembrane ephrinBs transduce reverse signaling in a tyrosine phosphorylation-dependent or -independent, as well as PDZ-dependent manner. Here, we show that ephrinB1 interacts with Connector Enhancer of KSR1 (CNK1) in an EphB receptor-independent manner. In cultured cells, cotransfection of ephrinB1 with CNK1 increases JNK phosphorylation. EphrinB1/CNK1-mediated JNK activation is reduced by overexpression of dominant-negative RhoA. Overexpression of CNK1 alone is sufficient for activation of RhoA; however, both ephrinB1 and CNK1 are required for JNK phosphorylation. Co-immunoprecipitation data showed that ephrinB1 and CNK1 act as scaffold proteins that connect RhoA and JNK signaling components, such as p115RhoGEF and MKK4. Furthermore, adhesion to fibronectin or active Src overexpression increases ephrinB1/CNK1 binding, whereas blocking Src activity by a pharmacological inhibitor decreases not only ephrinB1/CNK1 binding, but also JNK activation. EphrinB1 overexpression increases cell motility, however, CNK1 depletion by siRNA abrogates ephrinB1-mediated cell migration and JNK activation. Moreover, Rho kinase inhibitor or JNK inhibitor treatment suppresses ephrinB1-mediated cell migration. Taken together, our findings suggest that CNK1 is required for ephrinB1-induced JNK activation and cell migration.

G protein-coupled receptor kinase 2 (GRK2) is a Rho-activated scaffold protein for the ERK MAP kinase cascade.

The G protein-coupled receptor kinases (GRKs) are best known for their role in phosphorylating and desensitising G protein-coupled receptors (GPCRs). The GRKs also regulate signalling downstream of other families of receptors and have a number of non-receptor substrates and binding partners. Here we identify RhoAGTP and Raf1 as novel binding partners of GRK2 and report a previously unsuspected function for this kinase. GRK2 is a RhoA effector that serves as a RhoA-activated scaffold protein for the ERK MAP kinase cascade. The ability of GRK2 to bind to Raf1, MEK1 and ERK2 is dependent on RhoAGTP binding to the catalytic domain of the kinase. Exogenous GRK2 has previously been shown to increase ERK activation downstream of the epidermal growth factor receptor (EGFR). Here we find that GRK2-mediated ERK activation downstream of the EGFR is Rho-dependent and that treatment with EGF promotes RhoAGTP binding and ERK scaffolding by GRK2. Depletion of GRK2 expression by RNAi reveals that GRK2 is required for EGF-induced, Rho- and ERK-dependent thymidine incorporation in vascular smooth muscle cells (VSMCs). We therefore hypothesise that Rho-dependent ERK MAPK scaffolding by GRK2 downstream of the EGFR may have an important role in the vasculature, where increased levels of both GRK2 and RhoA have been associated with hypertension.