FKBPL and FKBP8 regulate DLK degradation and neuronal responses to axon injury
DLK is a key regulator of axon regeneration and degeneration in response to neuronal injury. To understand the molecular mechanisms controlling the DLK function, we performed yeast two-hybrid screening analysis and identified FKBPL as a DLK-binding protein that bound to the kinase domain and inhibited the kinase enzymatic activity of DLK. FKBPL regulated DLK stability through ubiquitin-dependent DLK degradation. We tested other members in the FKBP protein family and found that FKBP8 also induced DLK degradation as FKBPL did. We found that Lysine 271 residue in the kinase domain of DLK was a major site of ubiquitination and SUMO3-conjugation and responsible for FKBP8-mediated degradation. In vivo overexpression of FKBP8 delayed progression of axon degeneration and neuronal death following axotomy in sciatic and optic nerves, respectively, although axon regeneration efficiency was not enhanced. This research identified FKBPL and FKBP8 as new DLK-interacting proteins that regulated DLK stability by MG-132 or bafilomycin A1-sensitive protein degradation.