[show abstract][hide abstract] ABSTRACT: SCP/TAPS proteins are a diverse family of molecules in eukaryotes, including parasites. Despite their abundant occurrence in parasite secretomes, very little is known about their functions in parasitic nematodes, including blood-feeding hookworms. Current information indicates that SCP/TAPS proteins (called Ancylostoma-secreted proteins, ASPs) of the canine hookworm, Ancylostoma caninum, represent at least three distinct groups of proteins. This information, combined with comparative modelling, indicates that all known ASPs have an equatorial groove that binds extended structures, such as peptides or glycans. To elucidate structure-function relationships, we explored the three-dimensional crystal structure of an ASP (called Ac-ASP-7), which is highly up-regulated in expression in the transition of A. caninum larvae from a free-living to a parasitic stage. The topology of the N-terminal domain is consistent with pathogenesis-related proteins, and the C-terminal extension that resembles the fold of the Hinge domain. By anomalous diffraction, we identified a new metal binding site in the C-terminal extension of the protein. Ac-ASP-7 is in a monomer-dimer equilibrium, and crystal-packing analysis identified a dimeric structure which might resemble the homo-dimer in solution. The dimer interaction interface includes a novel binding site for divalent metal ions, and is proposed to serve as a binding site for proteins involved in the parasite-host interplay at the molecular level. Understanding this interplay and the integration of structural and functional data could lead to the design of new approaches for the control of parasitic diseases, with biotechnological outcomes.
[show abstract][hide abstract] ABSTRACT: Plant annexins show distinct differences in comparison with their animal orthologues. In particular, the endonexin sequence, which is responsible for coordination of calcium ions in type II binding sites, is only partially conserved in plant annexins. The crystal structure of calcium-bound cotton annexin Gh1 was solved at 2.5 A resolution and shows three metal ions coordinated in the first and fourth repeat in types II and III binding sites. Although the protein has no detectable affinity for calcium in solution, in the presence of phospholipid vesicles, we determined a stoichiometry of four calcium ions per protein molecule using isothermal titration calorimetry. Further analysis of the crystal structure showed that binding of a fourth calcium ion is structurally possible in the DE loop of the first repeat. Data from this study are in agreement with the canonical membrane binding of annexins, which is facilitated by the convex surface associating with the phospholipid bilayer by a calcium bridging mechanism. In annexin Gh1, this membrane-binding state is characterized by four calcium bridges in the I/IV module of the protein and by direct interactions of several surface-exposed basic and hydrophobic residues with the phospholipid membrane. Analysis of the protein fold stability revealed that the presence of calcium lowers the thermal stability of plant annexins. Furthermore, an additional unfolding step was detected at lower temperatures, which can be explained by the anchoring of the N-terminal domain to the C-terminal core by two conserved hydrogen bonds.
Journal of Biological Chemistry 07/2008; 283(26):18314-22. · 4.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Prohibitins comprise a family of highly conserved ubiquitous eukaryotic proteins that mainly localize to the mitochondria. They have been implicated in important cellular processes such as cellular signaling and transcriptional control, apoptosis, cellular senescence, and mitochondrial biogenesis. Using molecular modeling techniques, we have generated structural models of human prohibitins BAP32 and BAP37, which have previously been shown to exist as large ringlike oligomers in the membrane-bound state. The middle domain of prohibitins is evolutionary conserved in the family of SPFH (PHB) domain proteins. On the basis of the known structure of flotillin-2, another member of the SPFH-domain family, we have generated homology models for BAP32 and BAP37, and elucidated the implications for formation of high molecular weight oligomers. A model for the dimeric-building block of BAP32: BAP37 for such assemblies was generated and its stability scrutinized by molecular dynamics simulations. The model of BAP32 was also analyzed as to potential ligand-binding sites and the previously identified ligand melanogenin was docked into a membrane-proximal cavity. The results are discussed in the context of prohibitin interactions with mitochondrial AAA-proteases and we suggest two possible interaction interfaces between the BAP32:BAP37 building block and the protease.
Proteins Structure Function and Bioinformatics 07/2007; 68(1):353-62. · 3.34 Impact Factor
[show abstract][hide abstract] ABSTRACT: Annexin B1 from Cysticercus cellulosae has recently been identified using immunological screening in an attempt to find novel antigens for vaccine development against cysticercosis. The protein possesses anticoagulant activity and carries significant therapeutic potential due to its thrombus-targeting and thrombolytic properties. We investigated the biochemical properties of annexin B1 using liposome and heparin Sepharose copelleting assays, as well as CD spectroscopy. The calcium-dependent binding to acidic phospholipid membranes is reminiscent of other mammalian annexins with a clear preference for high phosphatidylserine content. A unique property of annexin B1 is its ability to bind to liposomes with high phosphatidylserine content in the absence of calcium, which might be due to the presence of several basic residues on the convex protein surface that harbours the membrane-binding loops. Annexin B1 demonstrates lectin properties and binds to heparin Sepharose in a cooperative, calcium-dependent manner. Although this binding is reversible to a large extent, a small fraction of the protein remains bound to the glycosaminoglycan even in the presence of high concentrations of EDTA. Analogous to annexin A5, we propose a model of heparin wrapped around the protein thereby engaging in calcium-dependent and calcium-independent interactions. Although the calcium-independent heparin-binding sites identified in annexin A5 are not conserved, we hypothesize three possible sites in annexin B1. Results from CD spectroscopy and thermal denaturation indicate that, in solution, the protein binds calcium with a low affinity that leads to a slight increase in folding stability.
[show abstract][hide abstract] ABSTRACT: The interactions of two plant annexins, annexin 24(Ca32) from Capsicum annuum and annexin Gh1 from Gossypium hirsutum, with phospholipid membranes have been characterized using liposome-based assays and adsorption to monolayers. These two plant annexins show a preference for phosphatidylserine-containing membranes and display a membrane binding behavior with a half-maximum calcium concentration in the sub-millimolar range. Surprisingly, the two plant annexins also display calcium-independent membrane binding at levels of 10-20% at neutral pH. This binding is regulated by three conserved surface-exposed residues on the convex side of the proteins that play a pivotal role in membrane binding. Due to quantitative differences in the membrane binding behavior of N-terminally His-tagged and wild-type annexin 24(Ca32), we conclude that the N-terminal domain of plant annexins plays an important role, reminiscent of the findings in their mammalian counterparts. Experiments elucidating plant annexin-mediated membrane aggregation and fusion, as well as the effect of these proteins on membrane surface hydrophobicity, agree with findings from the membrane binding experiments. Results from electron microscopy reveal elongated rodlike assemblies of plant annexins in the membrane-bound state. It is possible that these structures consist of protein molecules directly interacting with the membrane surface and molecules that are membrane-associated but not in direct contact with the phospholipids. The rodlike structures would also agree with the complex data from intrinsic protein fluorescence. The tubular lipid extensions suggest a role in the membrane cytoskeleton scaffolding or exocytotic processes. Overall, this study demonstrates the importance of subtle changes in an otherwise conserved annexin fold where these two plant annexins possess distinct modalities compared to mammalian and other nonplant annexins.