SummaryProtein tyrosine phosphatases (PTPs) play a critical role in regulating cellular functions by selectively dephosphorylating their substrates. Here we present 22 human PTP crystal structures that, together with prior structural knowledge, enable a comprehensive analysis of the classical PTP family. Despite their largely conserved fold, surface properties of PTPs are strikingly diverse. A potential secondary substrate-binding pocket is frequently found in phosphatases, and this has implications for both substrate recognition and development of selective inhibitors. Structural comparison identified four diverse catalytic loop (WPD) conformations and suggested a mechanism for loop closure. Enzymatic assays revealed vast differences in PTP catalytic activity and identified PTPD1, PTPD2, and HDPTP as catalytically inert protein phosphatases. We propose a “head-to-toe” dimerization model for RPTPγ/ζ that is distinct from the “inhibitory wedge” model and that provides a molecular basis for inhibitory regulation. This phosphatome resource gives an expanded insight into intrafamily PTP diversity, catalytic activity, substrate recognition, and autoregulatory self-association.

Background: Mutations in the ALK2 kinase cause extraskeletal bone formation.Results: We solved the structure of ALK2 in complex with the inhibitor FKBP12.Conclusion: Disease mutations break critical interactions that stabilize the inactive ALK2-FKBP12 complex leading to kinase activation.Significance: We offer an explanation for the effects of mutation and a structural template for the design of small molecule inhibitors.

f Constitutively activating mutations in receptor kinases recruit downstream effector pathways independently of upstream signaling, with consequences ranging from developmental syndromes to cancer. Classic fibrodysplasia ossificans progressiva (FOP) is a congenital syndrome resulting from highly conserved activating mutations of the glycine-serine-rich (GS) regulatory domain of ACVR1, encoding bone morphogenetic protein (BMP) type I receptor ALK2, which lead to inappropriate signaling and heterotopic ossification of soft tissues. It is unclear if constitutively active mutant ALK2 receptors (caALK2) can function independently of signaling complexes with type II receptors and ligands. We found that ablation of BmpRII and ActRIIa abrogated BMP ligand-mediated and caALK2-mediated signaling and transcription in cells and disrupted caALK2-induced heterotopic ossification in mice. Signaling via GS domain ALK2 mutants could be restored by the expression of either BMP type II receptor. The contribution of BMP type II receptors was independent of their ligand-binding or kinase function but was dependent upon an intact cytoplasmic domain. These data demonstrate that GS domain ALK2 mutants act independently of upstream signaling but may require a nonenzymatic scaffolding function provided by type II receptors to form functional, apparently ligand-independent signaling complexes. These findings define the minimal requirements for signaling of GS domain ALK2 mutants, with implications for the therapeutic targeting of their activity in disease. In human disease, somatic or germ line mutations may confer "constitutive" activity to mutant signaling or receptor proteins, permitting the recruitment of downstream effectors independently of upstream signaling events. So-called "constitutively activating" mutations and their gene products contribute to a broad range of clinical conditions ranging from cancer to syndromic developmental defects, including inborn disorders of metabolism, endocrine abnormalities, and sensory defects (1-3). Constitutively activating mutations, commonly referred to as "gain-offunction" mutations, can augment function in multiple ways. Constitutively active mutant receptor proteins may have true constitutive activity that is invariant with stimuli, harbor activity that is hypersensitive or amplified in response to native ligands, or have activity that can be induced with decreased specificity (4).Fibrodysplasia ossificans progressiva (FOP) is a congenital heterotopic ossification (HO) disorder in which afflicted individuals are nearly normal at birth except for subtle skeletal malformations yet are prone to forming endochondral bone lesions in soft tissues such as skeletal muscle, ligaments, and fascia, especially following injury or inflammation (5). The classic FOP phenotype results from highly conserved activating mutations in ACVR1, encoding the bone morphogenetic protein (BMP) type I receptor kinase ALK2, due to an R206H substitution in the glycine-serine-rich (GS) regulatory domain (6, 7). While other...