Identification of SIRT4 as a novel paralog-specific interactor and candidate suppressor of C-RAF kinase in MAPK signaling

Cellular responses leading to development, proliferation, and differentiation rely on RAF/MEK/ERK signaling that integrates and amplifies signals from various stimuli to cellular downstream responses. The clinical significance of C-RAF activation has been reported in many types of tumor cell proliferation and developmental disorders, which requires the discovery of potential C-RAF protein regulators. Here, we identify a novel and specific protein interaction between C-RAF, among the RAF kinase paralogs, and SIRT4 among the mitochondrial sirtuin family members SIRT3, SIRT4, and SIRT5. Structurally, C-RAF binds to SIRT4 through the N-terminal cysteine-rich domain (CRD; a.a. 136-187), and on the other side, SIRT4 requires predominantly the C-terminus (a.a. 255-314) for full interaction with C-RAF. Interestingly, SIRT4 interacts specifically with C-RAF in a pre-signaling inactive (serine 259 phosphorylated) state. Consistent with this finding, ectopic expression of SIRT4 in HEK293 cells results in upregulation of pS259-C-RAF levels and concomitant reduction of MAPK signaling as evidenced by strongly decreased phospho-ERK signals. Thus, our findings propose another extra-mitochondrial role of SIRT4 and suggest that SIRT4 functions as a cytosolic tumor suppressor of C-RAF-MAPK signaling, besides its known metabolic tumor suppressor role towards glutamate dehydrogenase and glutamine levels in mitochondria.