Revealing Activation Mechanism of Alk2 Kinase Mutations in Fibrodysplasia Ossificans Progressiva (FOP)

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Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder resulting in transformation of soft tissue into episodic bone formation. Most of patients die around age 40 and no effective therapy is available. The causative genetic mutations have been identified in the intracellular glycine-serine-rich (GS) domain and kinase domain of Activin-like kinase-2 (ALK2, also known as ACVR1), a type I receptor of bone morphogenetic protein (BMP)1. Cumulative studies support that these mutations abnormally activate BMP signaling in a ligand- dependent or independent manner. The molecular mechanisms underlying FOP remains unclear. To test a long-standing hypothesis that FOP mutations reducing the ALK2 interaction with the negative regulator FKBP12 protein, we performed extensive molecular dynamic studies of ALK2 systems. Six systems were simulated in explicit solvent for at least 300 nanoseconds each: ALK2WT, ALK2Q207E, ALK2R206H, and ALK2R206H+ (with protonated histidine), in complex with FKBP12, as well as phosphorylated ALK2 GS-domain with and without FKBP12.Our results show that both the phosphorylated and mutated ALK2 break a major inhibitory Arg375-Asp354 salt-bridge between A-loop and DLG motif, which initiates A-loop conformation change from DLG-in toward DLG-out. But in wild-type ALK2, this salt-bridge remains intact throughout 300 ns trajectory. Furthermore, the dissociation of FKBP12 triggers the formation of a Glu248-Lys235 salt-bridge between αC helix and β3 strand. Altogether, we conclude that the R206H promotes dissociation of FKBP from ALK2 and triggers αC helix swing toward the catalytic domain, which indicates the kinase shifting towards active conformation. Interestingly, Q207E model can trigger the formation of Glu248-Lys235 salt bridge (αC swing) without the FKBP dissociation. This finding well explains the different responses to inhibitory FKBP12 among FOP mutations in the reported luciferase assay for ALK2 activity2.1. Hum Mutat 30:3792. J Biol Chem 287:36990