Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

Abstract

The Ras\Raf\MEK (mitogen-activated protein kinase\ERK kinase)\ERK (extracellular-signal-regulated kinase) pathway is at the heart of signalling networks that govern proliferation, differentiation and cell survival. Although the basic regulatory steps have been elucidated, many features of this pathway are only beginning to emerge. This review focuses on the role of protein-protein interactions in the regulation of this pathway, INTRODUCTIONThe mitogen-activated protein kinase (MAPK) pathway is one of the primordial signalling systems that Nature has used in several permutations to accomplish an amazing variety of tasks. It exists in all eukaryotes, and controls such fundamental cellular processes as proliferation, differentiation, survival and apoptosis. The basic arrangement includes a G-protein working upstream of a core module consisting of three kinases : a MAPK kinase kinase (MAPKKK) that phosphorylates and activates a MAPK kinase (MAPKK), which in turn activates MAPK ( Figure 1). This set-up provides not only for signal amplification, but, maybe even more importantly, for additional regulatory interfaces that allow the kinetics, duration and amplitude of the activity to be precisely tuned. At present we can distinguish six MAPK modules, which share structurally related components, but seem to mediate specific biological responses. For recent reviews on MAPK pathways, the reader is referred to references [1][2][3]. Here we will focus on the regulation of the ERK (extracellular-signal-regulated kinase) pathway, which features Ras as G-protein, Raf as MAPKKK, MEK (MAPK\ERK kinase) as MAPKK and ERK as MAPK.Despite enjoying a decade in the limelight of scientific interest and revealing a plethora of new insights into the circuitry of signalling pathways in general, this pathway still holds many secrets. These pertain mainly to the regulation of Raf, and to a deeper understanding of how specific biological responses are encoded by spatial and temporal changes in the activity and subcellular distribution of the pathway components, and how these fluctuations are orchestrated at the molecular level. Work from many laboratories has highlighted protein-protein interactions as powerful means of co-ordinating signalling processes, most strikingly exemplified by the assembly of multi-protein signalling complexes on activated receptors, or of transcriptionfactor complexes on gene promoters. However, it is becoming Abbreviations used : Bcr, Breakpoint cluster region ; BXB, isolated Raf-1 kinase domain ; CNK, connector-enhancer of KSR ; CK2, casein kinase 2 ; CRD, cysteine-rich domain ; DEF, docking site for ERK, FxFP ; DLK, dual leucine-zipper-bearing kinase ; ERK, extracellular-signal-regulated kinase ; Hsp, heat-shock protein ; KIM, kinase interaction motif ; IκB, inhibitor of NF-κB ; JNK, c-Jun N-terminal kinase ; KSR, kinase suppressor of Ras ; MAPK, mitogen-activated protein kinase ; MAPKK, MAPK kinase ; MAPKKK, MAPK kinase kinase ; MEK, MAPK/ERK kinase ; MEKK, MEK kinase ; MP1, MEK partner 1 ; MUK, MAPK upstream kin...

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