Hsp90-Binding Immunophilin FKBP51 is a Phosphoprotein Differentially Modified During Neuronal Differentiation

C DANERI, N ZGAJNAR, C LOTUFO, A CAUERHFF, MD GALIGNIANA

Nottingham

Congreso; Life Sciences 2019: Post-Translational modifications and cell signalling; 2019

Immunophilins are a family of peptidylprolyl-isomerases that bind immunosuppressive ligands. FK506-Binding Proteins (FKBPs) group a subfamily that binds the macrolide FK506 (tacrolimus). Immunosuppression is mediated by low M.W. immunophilins, but not by the high M.W. counterparts, whose biological functions are poorly understood to date. Previously, we reported that FKBP51 (the 51-kDa FKBP): a)-is a novel mitochondrial factor that undergoes dynamic nuclear-mitochondrial shuttling, b)-shows antiapoptotic and pro-tumoral features, and c)-is abundant in the nervous system. In this study it is demonstrated that FKBP51 shows five isoforms resolved by SDS/PAGE. Lower mobility bands undergo a gel-shift to the fastest mobility band after preincubation with alkaline phosphatase, suggesting that they are phosphorylated isoforms. In all cell types, each FKBP51 isoform specifically localizes in nuclei, mitochondria or cytosol according to the phosphorylation status. Specific anti-phospho-amino acid antibodies revealed that P-Thr is the main phospho-amino acid residue in both mesoderm- and endoderm-derived cell types, but P-Tyr and P-Ser prime in neurons. When undifferentiated neuronal cells are incubated with FK506 (and no trophic factors in the medium, including serum), they rapidly acquire a neuron phenotype whereas P-Tyr signal is enhanced and P-Ser signal fades. Structural analysis of FKBP51 reveals the presence of specific phosphorylated residues in its peptidylprolyl-isomerase domain, this post-translational modification being stimulated upon FK506 binding. This is the first study showing that FKBP51 is a phosphoprotein and the biological significance of this post-translational modification.