KSR is a scaffold required for activation of the ERK/MAPK module

Abstract

Mechanisms that regulate signal propagation through the ERK/MAPK pathway are still poorly understood. Several proteins are suspected to play critical roles in this process. One of these is Kinase Suppressor of Ras (KSR), a component previously identified in RAS-dependent genetic screens in Drosophila and Caenorhabditis elegans. Here, we show that KSR functions upstream of MEK within the ERK/MAPK module. In agreement with this, we found that KSR facilitates the phosphorylation of MEK by RAF. We further show that KSR associates independently with RAF and MEK, and that these interactions lead to the formation of a RAF/MEK complex, thereby positioning RAF in close proximity to its substrate MEK. These findings suggest that KSR functions as a scaffold that assembles the RAF/MEK functional pair.

Signal control through Raf: in sickness and in health

“…KSR1 may be most important as a scaffold due to its ability to bind simultaneously to components at each level of the MAPK cascade (MAP3K, MAP2K, and MAPK). Because signaling is diminished if it is expressed in excess of its binding partners, KSR1 fits the characteristics of a scaffold [27,30,32]. A functional catalytic lysine appears to be absent from KSR, unlike what we found in WNK protein kinases; in WNKs the catalytic lysine is not in the canonical position, but it is nearby in a distinct structural element [33].…”

“…The relevance of the catalytic activity of KSR proteins is in debate [27][28][29][30][31]. KSRs contain arginine instead of the invariant catalytic lysine in the N-terminal region of the kinase domain present in other protein kinases.…”

Mitogen-Activated Protein Kinases and Reactive Oxygen Species Signaling in Plants

“…This raises the question of how specificity of a certain MAPK-mediated response can be achieved. Scaffolding proteins that support the formation of a particular MAPKKK-MAPKK-MAPK complex are known from yeast (Saccharomyces cerevisiae; O'Rourke and Herskowitz, 1998) and animals (Roy et al, 2002). MAPK modules with scaffolding function have also recently been identified in plants.…”

Nuclear Export and Plasma Membrane Recruitment of the Ste5 Scaffold Are Coordinated with Oligomerization and Association with Signal Transduction Components

“…Fus3 is predominantly nuclear, whereas Ste7 and Ste11 are cytoplasmic van Drogen et al, 2001), raising the possibility that nuclear shuttling helps Ste5 to assemble a signaling complex. Interestingly, the kinase suppressor of Ras scaffold also links MAPK cascade kinases (Raf, extracellular signal-regulated kinase kinase, extracellular signal-regulated kinase) to a membranelinked anchor (Ras) (Nguyen et al, 2002;Roy et al, 2002) and shuttles through the nucleus (Brennan et al, 2002), suggesting that multiple aspects of Ste5 localization are conserved.…”

“…The Ste5 scaffold of budding yeast is the prototype of a family of proteins that tether kinases of mitogen-activated protein kinase (MAPK) cascades in eukaryotes (Elion, 1995(Elion, , 2001Burack and Shaw, 2000;Nguyen et al, 2002;Roy et al, 2002). Ste5 assembles the mating pathway kinases Ste11 (MAPKKK), Ste7 (MAPKK), and Fus3 (MAPK) into an active signaling complex of high specific activity (Kranz et al, 1994;Choi et al, 1994; by binding the kinases through separate binding sites ( Figure 1A; Choi et al, 1994;Inouye et al, 1997b).…”