Isoform-specific localization of A-RAF in mitochondria

RAF kinase is a family of isoforms including A-RAF, B-RAF, and C-RAF, Despi te the important role of RAF in cell growth and proliferation, little evide nce exists for isoform-specific function of RAF family members. Using Weste rn analysis and immunogold labeling, A-RAF was selectively localized in hig hly purified rat liver mitochondria. Two novel human proteins, which intera ct specifically with A-RAF, were identified, and the full-length sequences are reported. These proteins, referred to as hTOM and hTIM, are similar to components of mitochondrial outer and inner membrane protein-import recepto rs from lower organisms, implicating their involvement in the mitochondrial transport of A-RAF. hTOM contains multiple tetratricopeptide repeat (TPR) domains, which function in protein-protein interactions. TPR domains are fr equently present in proteins involved in cellular transport systems. In con trast, protein 14-3-3, an abundant cytosolic protein that participates in m any facets of signal transduction, was found to interact with C-RAF but not with A-RAF N-terminal domain. This information is discussed in view of the important role of mitochondria in cellular functions involving energy bala nce, proliferation, and apoptosis and the potential role of A-RAF in regula ting these systems.