Raf‐1 and B‐Raf proteins have similar regional distributions but differential subcellular localization in adult rat brain

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Abstract
The Raf kinases play an important and specific role in the activation of extracellular signal‐regulated kinases (ERK) cascade. Beside its role in the control of proliferation and differentiation, the ERK cascade has also been implicated in neuron‐specific functions. In order to gain clues on the function of Raf kinases in the adult central nervous system (CNS), we performed a comparative analysis of the distribution and subcellular localization of the different Raf kinases in rat brain with antibodies specific for the different Raf kinases. We show that B‐Raf and Raf‐1 proteins are present in most brain areas, whereas A‐Raf is not detected. Interestingly, the two Raf proteins have an approximately similar pattern of distribution with a rostro‐caudal decreasing gradient of expression. These two kinases are colocalized in neurons but they are differentially located in subcellular compartments. Raf‐1 is localized mainly in the cytosolic fraction around the nucleus, whereas B‐Raf is widely distributed in the cell bodies and in the neuritic processes. In addition, we demonstrated that numerous B‐Raf isoforms are present in the brain. These isoforms have a differential pattern of distribution, some of them being ubiquitously expressed whereas others are localized to specific brain areas. These isoforms also have a clear differential subcellular localization, specially in Triton‐insoluble fractions, but also in synaptosomal, membrane and cytosolic compartments. Altogether these results suggest that each Raf protein could have a distinct signalling regulatory function in the brain with regard to its subcellular localization.

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