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In this issue of Structure, Gupta and colleagues apply a combination of biophysical approaches to study the solution properties of prototype foamy virus (PFV) integrase alone and in complex with viral DNA ends (intasome). The results complement and extend previous structural studies of PFV intasomes by X-ray crystallography and highlight the synergy of solution and crystallographic approaches to the study of nucleoprotein complexes., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Using small-angle X-ray and neutron scattering (SAXS/SANS), in combination with analytical centrifugation and light scattering, we have determined the solution properties of PFV IN alone and its synaptic complex with processed U5 viral DNA and related these properties to models derived from available crystal structures. PFV IN is a monomer in solution, and SAXS analysis indicates an ensemble of conformations that differ from that observed in the crystallographic DNA-bound state. Scattering data indicate that the PFV intasome adopts a shape in solution that is consistent with the tetrameric assembly inferred from crystallographic symmetry, and these properties are largely preserved in the presence of divalent ions and clinical strand transfer inhibitors. Using contrast variation methods, we have reconstructed the solution structure of the PFV intasome complex and have located the distal domains of IN that were unresolved by crystallography. These results provide important insights into the architecture of the retroviral intasome., (Copyright © 2012 Elsevier Ltd. All rights reserved.)