Identification of B-KSR1, a novel brain-specific isoform of KSR1 that functions in neuronal signaling

Kinase suppressor of Ras (KSR) is an evolutionarily conserved component of Ras-dependent signaling pathways. Here, we report the identification of E-K SR1, a novel splice variant of murine KSR1 that is highly expressed in brai n-derived tissues. B-KSR1 protein is detectable in mouse brain throughout e mbryogenesis, is most abundant in adult forebrain neurons, and is complexed with activated mitogen-activated protein kinase (MAPK) and MEK in brain ti ssues. Expression of B-KSR1 in PC12 cells resulted in accelerated nerve gro wth factor (NGF)-induced neuronal differentiation and detectable epidermal growth factor (EGF)-induced neurite outgrowth. Sustained MAPK activity was observed in cells stimulated with either NGF or EGF, and all effects on neu rite outgrowth could be blocked by the MEK inhibitor PD98059. In B-KSR1-exp ressing cells, the MAPK-B-KSR1 interaction was inducible and correlated wit h MAPK activation, while the MEK-B-KSR1 interaction was constitutive. Furth er examination of the MEK-B-KSR1 interaction revealed that all genetically identified loss-of-function mutations in the catalytic domain severely dimi nished MEK binding. Moreover, B-KSR1 mutants defective in MER binding were unable to augment neurite outgrowth. Together, these findings demonstrate t he functional importance of MEK binding and indicate that B-KSR1 may functi on to transduce Ras-dependent signals that are required for neuronal differ entiation or that are involved in the normal functioning of the mature cent ral nervous system.