Homology Modeling and Crystal Structure Determination of BMP Receptor Kinases

Objectives: To provide homology-based and experimentally determined models for interpretation of the structural and mechanistic basis of receptor kinase dysregulation due to mutation and for rational design of small molecule inhibitors of bone morphogenetic protein (BMP) signaling. Methods: To date only the 3-D structure of the kinase domain of the TGF-β type I receptor (TβRI) has been determined, out of the seven receptors of this family encoded within the human genome. Nevertheless, the TβRI kinase domain provides a sound structural template for generating reliable models of wildtype and mutated BMP receptor kinases based on their homologies. Furthermore, recent advances in bacterial co-expression of kinases of interest and appropriate phosphatases have rendered milligram scale production of homogeneous kinase preparations more tractable. Results: A TβRI-based homology model of ACVR1 (ALK-2), one of three BMP type I receptor kinases, has been produced along with eight models of mutant receptors linked to classic, atypical and variant forms of FOP (Fibrodysplasia Ossificans Progessiva), a severely disabling form of ectopic skeletogenesis. The results of homology modeling show that all of the mutations likely affect structural features of the kinase domain required for autoinhibition of the receptor in the absence of ligand. Work toward obtaining experimentally determined models by X-ray crystallography is currently underway through bacterial co-expression of wild-type and mutant ACVR1 receptor kinase domains and a multifunctional phosphatase. Conclusions: Structure-based homology modeling reveals structural and mechanistic bases for dysregulation of BMP signaling associated with mutations in the human receptors. Determination of crystal structures will provide models for design of ATP analog inhibitors (based on the Imatinib paradigm) for therapeutic applications in the treatment of FOP and other disorders linked to the BMP pathway. (Study supported in part by extramural funding from The Center for Research in FOP & Related Disorders at the University of Pennsylvania).