TGF-ß2 uses the concave surface of its extended finger region to bind betaglycan's ZP domain via three residues specific to TGF-ß and inhibin-α.

Betaglycan (BG) is a membrane-bound co-receptor of the TGF-ß family that selectively binds transforming growth factor-ß (TGF-ß) isoforms and inhibin A (InhA) to enable temporal-spatial patterns of signaling essential for their functions in vivo Here, using NMR titrations of methyl-labeled TGF-ß2 with BG's C-terminal binding domain, BGZP-C, and surface plasmon resonance binding measurements with TGF-ß2 variants, we found that the BGZP-C-binding site on TGF-ß2 is located on the inner surface of its extended finger region. Included in this binding site are Ile-92, Lys-97, and Glu-99, which are entirely or mostly specific to the TGF-ß isoforms and the InhA α-subunit, but they are unconserved in other TGF-ß family growth factors (GFs). In accord with the proposed specificity-determining role of these residues, BG bound bone morphogenetic protein 2 (BMP-2) weakly or not at all, and TGF-ß2 variants with the corresponding residues from BMP-2 bound BGZP-C more weakly than corresponding alanine variants. The BGZP-C-binding site on InhA previously was reported to be located on the outside of the extended finger region, yet at the same time to include Ser-112 and Lys-119, homologous to TGF-ß2 Ile-92 and Lys-97, on the inside of the fingers. Therefore, it is likely that both TGF-ß2 and InhA bind BGZP-C through a site on the inside of their extended finger regions. Overall, these results identify the BGZP-C-binding site on TGF-ß2 and shed light on the specificity of BG for select TGF-ß-type GFs and the mechanisms by which BG influences their signaling.

Structure of the Alk1 extracellular domain and characterization of its bone morphogenetic protein (BMP) binding properties.

Bone morphogenetic proteins (BMPs) are secreted signaling proteins - they transduce their signals by assembling complexes comprised of one of three known type II receptors and one of four known type I receptors. BMP-9 binds and signals through the type I receptor Alk1, but not other Alks, while BMP-2, -4, and -7 bind and signal through Alk3, and the close homologue Alk6, but not Alk1. The present results, which include the determination of the Alk1 structure using NMR and identification of residues important for binding using SPR, show that the ß-strand framework of Alk1 is highly similar to Alk3, yet there are significant differences in loops shown previously to be important for binding. The most pronounced difference is in the N-terminal portion of the ß4-ß5 loop, which is structurally ordered and includes a similarly placed but shorter helix in Alk1 compared to Alk3. The altered conformation of the ß4-ß5 loop, and to lesser extent ß1-ß2 loop, cause clashes when Alk1 is positioned onto BMP-9 in the manner that Alk3 is positioned onto BMP-2. This necessitates an alternative manner of binding, which is supported by a model of the BMP-9/Alk1 complex constructed using the program RosettaDock. The model shows that Alk1 is positioned similar to Alk3 but is rotated by 40 deg. The alternate positioning allows Alk1 to bind BMP-9 through a large hydrophobic interface, consistent with mutational analysis that identified several residues in the central portion of the ß4-ß5 loop that contribute significantly to binding and are nonconservatively substituted relative to the corresponding residues in Alk3.

Effect of disease-linked mutations on the structure, function, stability and aggregation of human carbonic anhydrase II.

Point mutations in gene sequence often lead to protein misfolding or destabilization which is a well-known cause of a number of loss-of-function diseases. The carriers of point mutations in the human carbonic anhydrase II (HCAII) gene have been recognized to display carbonic anhydrase II deficiency syndrome (CADS). Two such single point mutations linked with CADS involve Gly145Arg and His94Tyr substitution. In the present study, we obtained these two single mutants of HCAII using site-directed mutagenesis, and successfully expressed and purified them. To examine the effect of mutations on the structure and function of HCAII, we carried out circular dichroism, intrinsic fluorescence, NMR measurements and activity assays. Studies suggest that the mutant proteins undergo local structural perturbations and have compromised native state stability. HCAIIH94Y (H94Y), being an active site mutant, shows larger destabilization effect as compared to HCAIIG145R (G145R). GdnHCl-denaturation studies showed that HCAII unfolding is a two-step process (N â‡Œ I â‡Œ U) and the free energy of first transition (N â‡Œ I) decreases by 1.5 kJ mol-1 and 4.9 kJ mol-1 for G145R and H94Y, respectively. Conformational changes and enzyme activity were established through various spectroscopic techniques.

An A1-A1 mutant with improved binding and inhibition of ß2GPI/antibody complexes in antiphospholipid syndrome.

ß2 glycoprotein I (ß2GPI) is the most common antigen for autoimmune antibodies in antiphospholipid syndrome (APS). Thrombosis is a clinical feature of APS. We created a molecule (A1-A1) that consists of two identical ß2GPI-binding modules from ApoE receptor 2 (ApoER2). A1-A1 binds to ß2GPI/antibody complexes, preventing their association with ApoER2 and anionic phospholipids, and reducing thrombus size in the mouse model of APS. Here, we describe a mutant of A1-A1 (mA1-A1ND) with improved affinity for ß2GPI. mA1-A1ND inhibits the binding of ß2GPI to cardiolipin in the presence of anti-ß2GPI antibodies, and inhibits the binding to phospholipids in plasma samples of APS patients, affecting the clotting time. Reduction of the clotting time demonstrates the presence of soluble ß2GPI/antibody complexes in patients' plasma. These complexes either already exist in patients' plasma or form rapidly in the proximity to phospholipids. All members of the low-density lipoprotein receptor family bind ß2GPI. Modeling studies of A1 in a complex with domain V of ß2GPI (ß2GPI-DV) revealed two possible modes of interaction of a ligand-binding module from lipoprotein receptors with ß2GPI-DV. In both orientations, the ligand-binding module interferes with binding of ß2GPI to anionic phospholipids; however, it interacts with two different but overlapping sets of lysine residues in ß2GPI-DV, depending on the orientation.

The TßR-I pre-helix extension is structurally ordered in the unbound form and its flanking prolines are essential for binding.

Transforming growth factor ß isoforms (TGF-ß) are among the most recently evolved members of a signaling superfamily with more than 30 members. TGF-ß play vital roles in regulating cellular growth and differentiation, and they signal through a highly restricted subset of receptors known as TGF-ß type I receptor (TßR-I) and TGF-ß type II receptor (TßR-II). TGF-ß's specificity for TßR-I has been proposed to arise from its pre-helix extension, a five-residue loop that binds in the cleft between TGF-ß and TßR-II. The structure and backbone dynamics of the unbound form of the TßR-I extracellular domain were determined using NMR to investigate the extension's role in binding. This showed that the unbound form is highly similar to the bound form in terms of both the ß-strand framework that defines the three-finger toxin fold and the extension and its characteristic cis-Ile54-Pro55 peptide bond. The NMR data further showed that the extension and two flanking 3(10) helices are rigid on the nanosecond-to-picosecond timescale. The functional significance of several residues within the extension was investigated by binding studies and reporter gene assays in cultured epithelial cells. These demonstrated that the pre-helix extension is essential for binding, with Pro55 and Pro59 each playing a major role. These findings suggest that the pre-helix extension and its flanking prolines evolved to endow the TGF-ß signaling complex with its unique specificity, departing from the ancestral promiscuity of the bone morphogenetic protein subfamily, where the binding interface of the type I receptor is highly flexible.