Solution structure of the carboxyl-terminal cysteine-rich domain of the VHv1.1 polydnaviral gene product: Comparison with other cystine knot structural folds

Polydnaviruses are an unusual group of insect viruses that have ail obligat e symbiotic association with certain parasitic wasps. These viruses are tra nsmitted with the wasp egg during oviposition into lepidopteran insects, en abling the survival and development of the egg inside the host larvae. We r eport the three-dimensional structure of a novel polydnaviral cysteine-rich motif (cys-motif), identified as the carboxyl-terminal domain of a two cys -motif containing polydnaviral VHv 1.1 gone product, abbreviated "C-term VH v1.1". This 65-residue domain was identified experimentally by limited prot eolysis of the full-length protein and was subsequently cloned in a bacteri al expression system for NMR studies. The C-term VHv1.1 3D structure was de termined in solution by two-dimensional H-1 NMR spectroscopy. Calculation o f the structure was based on a total of 300 upper distance restraints and 2 0 dihedral angle constraints, and resulted in an ensemble of 25 representat ive conformers with an average rmsd of 0.47 A from the mean structure for c ore backbone atoms. The protein core is made of a four beta -strand scaffol d held together in a compact structure by three disulfide bonds, which form a cystine knot. The four beta -strands are arranged in an unusual configur ation to form a triple-stranded beta -shect and double-stranded beta -sheet . Comparison with other classes of cystine knots provides indication that C -term VHv 1.1 represents a new and distinct cytine knot motif. This analysi s provides a structural basis for interpretation of the genetic and amino a cid sequence data classifying polydnavirus gene products as members of cyst eine-rich protein families.