Editors' ChoiceMAPK PATHWAY

Ambivalent About Raf Activation

Science's STKE  07 Oct 2003:
Vol. 2003, Issue 203, pp. tw394-TW394
DOI: 10.1126/stke.2003.203.tw394

The activation of the small guanosine triphosphatase Ras leads to the recruitment to the plasma membrane of the serine-threonine kinase Raf during initiation of the extracellular-regulated kinase mitogen-activated protein kinase (MAPK) signaling pathway. The interaction with Ras is not sufficient for Raf activation; however, the nature of the other signals involved in Raf activation has been unclear. Douziech et al. used RNA interference (RNAi) together with transfection of signaling mutants to investigate the role of connector enhancer of KSR (CNK), a multidomain protein previously implicated in the MAPK pathway as a possible Raf activator. When S2 cells expressing constitutively active Ras or a constitutively active receptor tyrosine kinase were depleted of CNK by RNAi, phosphorylation of MAPK kinase (MEK) and MAPK were inhibited, whereas CNK depletion did not inhibit activation of MEK and MAPK in the presence of constitutively active Raf. The authors expressed CNK mutants together with constitutively active components of the MAPK pathway to determine that CNK, acting through distinct domains, had dual effects on MAPK activation. Two N-terminal domains, CRIC (conserved region in CNK) and SAM (sterile alpha motif) were required for Raf activation, whereas a 30-amino acid C-terminal region made up of two distinct elements inhibited Raf activation of MEK. Thus, CNK appears to have a bimodal effect on signaling through the MAPK pathway, with opposing effects upstream and downstream of Raf. The authors propose a model whereby CNK prevents signaling from Raf under resting conditions but facilitates Raf activation by Ras.

M. Douziech, F. Roy, G. Laberge, A.-V. Armengod, M. Therrien, Bimodal regulation of RAF by CNK in Drosophila. EMBO J. 22, 5068-5078 (2003). [Abstract] [Full Text]