TGF-beta ligands stimulate diverse cellular differentiation and growth responses by signaling through type I and II receptors. Ligand antagonists, such as follistatin, block signaling and are essential regulators of physiological responses. Here we report the structure of activin A, a TGF-beta ligand, bound to the high-affinity antagonist follistatin. Two follistatin molecules encircle activin, neutralizing the ligand by burying one-third of its residues and its receptor binding sites. Previous studies have suggested that type I receptor binding would not be blocked by follistatin, but the crystal structure reveals that the follistatin N-terminal domain has an unexpected fold that mimics a universal type I receptor motif and occupies this receptor binding site. The formation of follistatin:BMP:type I receptor complexes can be explained by the stoichiometric and geometric arrangement of the activin:follistatin complex. The mode of ligand binding by follistatin has important implications for its ability to neutralize homo- and heterodimeric ligands of this growth factor family.

Adeno-associated viruses (AAVs) are leading candidate vectors for human gene therapy. AAV serotypes have broad cellular tropism and use a variety of cellular receptors. AAV serotype 3 binds to heparan sulfate proteoglycan prior to cell entry and is serologically distinct from other serotypes. The capsid features that distinguish AAV-3B from other serotypes are poorly understood. The structure of AAV-3B has been determined to 2.6Å resolution from twinned crystals of an infectious virus. The most distinctive structural features are located in regions implicated in receptor and antibody binding, providing insights into the cell entry mechanisms and antigenic nature of AAVs. We show that AAV-3B has a lower affinity for heparin than AAV-2, which can be rationalized by the distinct features of the AAV-3B capsid. The structure of AAV-3B provides an additional foundation for the future engineering of improved gene therapy vectors with modified receptor binding or antigenic characteristics.

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The transforming growth factor-b (TGF-b) superfamily is a large group of structurally related growth factors that play prominent roles in a variety of cellular processes. The importance and prevalence of TGF-b signaling are also reflected by the complex network of check points that exist along the signaling pathway, including a number of extracellular antagonists and membranelevel signaling modulators. Recently, a number of important TGF-b crystal structures have emerged and given us an unprecedented clarity on several aspects of the signal transduction process. This review will highlight these latest advances and present our current understanding on the mechanisms of specificity and regulation on TGF-b signaling outside the cell.

Crystal structures of the AAV-6 capsid at 3 Å reveal a subunit fold homologous to other parvoviruses with greatest differences in two external loops. The electrostatic potential suggests that receptor-attachment is mediated by four residues: Arg576, Lys493, Lys459 and Lys531, defining a positively charged region curving up from the valley between adjacent spikes. It overlaps only partially with the receptor-binding site of AAV-2, and the residues endowing the electrostatic character are not homologous. Mutational substitution of each residue decreases heparin affinity, particularly Lys531 and Lys459. Neither is conserved among heparin-binding serotypes, indicating that diverse modes of receptor attachment have been selected in different serotypes. Surface topology and charge are also distinct at the shoulder of the spike, where linear epitopes for AAV-2’s neutralizing monoclonal antibody A20 come together. Evolutionarily, selection of changed side-chain charge may have offered a conservative means to evade immune neutralization while preserving other essential functionality.

Adeno-associated virus is a promising vector for gene therapy. In the current study, the binding site on AAV serotype 3B for the heparan sulfate proteoglycan (HSPG) receptor has been characterized. X-ray diffraction identified a disaccharide binding site at the most positively charged region on the virus surface. The contributions of basic amino acids at this and other sites were characterized using site-directed mutagenesis. Both heparin and cell binding are correlated to positive charge at the disaccharide binding site, and transduction is significantly decreased in AAV-3B vectors mutated at this site to reduce heparin binding. While the receptor attachment sites of AAV-3B and AAV-2 are both in the general vicinity of the viral spikes, the exact amino acids that participate in electrostatic interactions are distinct. Diversity in the mechanisms of cell attachment by AAV serotypes will be important considerations for the rational design of improved gene therapy vectors.

Follistatin (FS) regulates transforming growth factor-␤ superfamily ligands and is necessary for normal embryonic and ovarian follicle development. Follistatin is expressed as two splice variants (FS288 and FS315). Previous studies indicated differences in heparin binding between FS288 and FS315, potentially influencing the physiological functions and locations of these isoforms. We have determined the structure of the FS315-activin A complex and quantitatively compared heparin binding by the two isoforms. The FS315 complex structure shows that both isoforms inhibit activin similarly, but FS315 exhibits movements within follistatin domain 3 (FSD3) apparently linked to binding of the C-terminal extension. Surprisingly, the binding affinities of FS288 and FS315 for heparin are similar at lower ionic strengths with FS315 binding decreasing more sharply as a function of salt concentration. When bound to activin, FS315 binds heparin similarly to the FS288 isoform, consistent with the structure of the complex, in which the acidic residues of the C-terminal extension cannot interact with the heparin-binding site. Activin-induced binding of heparin is unique to the FS315 isoform and may stimulate clearance of FS315 complexes.Ligands of the transforming growth factor-␤ (TGF␤) 3 superfamily regulate a diverse set of cellular and physiological functions, including early embryonic development, cellular growth and proliferation, and reproduction (1-3). Activin A, a member of the TGF␤ superfamily, is necessary to the regulation of both the male and the female reproductive axes by stimulating the release of follicle-stimulating hormone from the pituitary (4, 5). Activin A also has other important physiological roles, because activin A-deficient mice die shortly after birth and exhibit multiple defects in craniofacial development (6, 7). Comparative studies of the similarities and differences between TGF␤ ligands have lead to a better understanding of the many physiological effects of this multipotent ligand superfamily.TGF␤ superfamily members typically exist as covalently linked dimers and fall into three main sub-categories: TGF␤s, activins/inhibins/nodals, and bone morphogenetic proteins (BMPs) (8). Structurally, these proteins all share a cysteine-knot fold and signal by engaging type II and type I receptors (9). Activin and BMPs bind to their type II receptors through a structural feature known as the "knuckle" region (10 -12). They have similar type I receptor binding sites on the concave surfaces of the molecule, which span both monomers of the ligands (13-16). Although specific ligand:receptor pairs have been characterized, multiple ligands share several receptors providing complexity and precise control to the system. The members of this superfamily are produced as prepro-proteins, and the pro-domain often regulates biological functions. In the absence of the pro-domain, TGF␤-1 and -2, as well as BMP-2, -4, and -7, can bind directly to the cell surface through heparin binding sites (17,18). This provides a mechanism f...

Adeno-associated virus (AAV) is a key candidate in the development of gene therapy. In this report, we used surface plasmon resonance spectroscopy to study the interaction between AAV and heparin and other glycosaminoglycans. Surface plasmon resonance results revealed that heparin binds to AAV with extremely high affinity. Solution competition studies shows that AAV binding to heparin is chain length dependent. AAV prefers to bind full chain heparin. All sulfo groups (especially N-sulfo and 6-O-sulfo groups) on heparin are important for the AAV- heparin interaction. Higher levels of sulfo group substitution in GAGs enhance their binding affinities. Atomic force microscopy was also performed to image AAV-2 complexed with heparin.