AKAP-Lbc enhances cyclic AMP control of the ERK1/2 cascade

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Abstract
Cyclic AMP is known to affect RAF–MEK–ERK signalling, but the precise mechanism has remained unknown. An interaction between AKAP-Lbc and the scaffolding protein KSR1 is now shown to tie cAMP production to ERK pathway regulation. Mitogen-activated protein kinase (MAPK) cascades propagate a variety of cellular activities1. Processive relay of signals through RAF–MEK–ERK modulates cell growth and proliferation2,3. Signalling through this ERK cascade is frequently amplified in cancers, and drugs such as sorafenib (which is prescribed to treat renal and hepatic carcinomas) and PLX4720 (which targets melanomas) inhibit RAF kinases4,5. Natural factors that influence ERK1/2 signalling include the second messenger cyclic AMP6,7. However, the mechanisms underlying this cascade have been difficult to elucidate. We demonstrate that the A-kinase-anchoring protein AKAP-Lbc and the scaffolding protein kinase suppressor of Ras (KSR-1) form the core of a signalling network that efficiently relay signals from RAF, through MEK, and on to ERK1/2. AKAP-Lbc functions as an enhancer of ERK signalling by securing RAF in the vicinity of MEK1 and synchronizing protein kinase A (PKA)-mediated phosphorylation of Ser 838 on KSR-1. This offers mechanistic insight into cAMP-responsive control of ERK signalling events.

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