Intermolecular interactions and characterization of the novel factor Xa exosite involved in macromolecular recognition and inhibition: crystal structure of human Gla-domainless factor Xa complexed with the anticoagulant protein NAPc2 from the hematophagous nematode Ancylostoma caninum.

NAPc2, an anticoagulant protein from the hematophagous nematode Ancylostoma caninum evaluated in phase-II/IIa clinical trials, inhibits the extrinsic blood coagulation pathway by a two step mechanism, initially interacting with the hitherto uncharacterized factor Xa exosite involved in macromolecular recognition and subsequently inhibiting factor VIIa (K(i)=8.4 pM) of the factor VIIa/tissue factor complex. NAPc2 is highly flexible, becoming partially ordered and undergoing significant structural changes in the C terminus upon binding to the factor Xa exosite. In the crystal structure of the ternary factor Xa/NAPc2/selectide complex, the binding interface consists of an intermolecular antiparallel beta-sheet formed by the segment of the polypeptide chain consisting of residues 74-80 of NAPc2 with the residues 86-93 of factor Xa that is additional maintained by contacts between the short helical segment (residues 67-73) and a turn (residues 26-29) of NAPc2 with the short C-terminal helix of factor Xa (residues 233-243). This exosite is physiologically highly relevant for the recognition and inhibition of factor X/Xa by macromolecular substrates and provides a structural motif for the development of a new class of inhibitors for the treatment of deep vein thrombosis and angioplasty.

Agroecotypes or phenotypic plasticity? comparison of agrestal and ruderal populations of the weed Solanum ptycanthum.

To test if the high nutrient inputs of agroeosystems select for specialized agroecotypes or for phenotypic plasticity, Ontario populations of the northwardly migrating annual weed Solanum ptycanthum from ruderal (beach) and agricultural habitats were compared over a nutrient gradient. Temporal variation of total available nitrogen was determined in both types of habitats. As gene flow via seed contamination of tomato transplants from S. United States was detected, variation in response to nutrient (N) levels was also compared between agrestal populations from the northern (Ontario) and southern (Georgia) ends of the species range. Five families from six populations (two northern agrestal, two northern ruderal and two southern agrestal) were grown in the greenhouse at low, medium and high nutrient levels, and plant growth and traits associated with reproductive success measured. All populations displayed significant levels of plasticity in the majority of vegetative and reproductive traits. There were no detectable differences over the levels of nutrients tested between individuals sampled from northern agrestal and ruderal populations, even though variation in available nitrogen is greater in agroecosystems. Southern agrestal populations were genetically differentiated from the northern populations, and exhibited almost twice the overall plasticity of northern populations, measured by the Mahalanobis distance. Canonical discriminant analysis showed complete overlap in the northern populations over all nutrient levels, suggesting that colonization of new habitats is via a general-purpose genotype, rather than by selection for specialized agroeotypes.