Retroviral intasomes arising

Abstract

Retroviral DNA integration takes place in the context of the intasome nucleoprotein complex. X-ray crystal structures of functional spumaviral intasomes were previously revealed to harbor a homotetramer of integrase, and it was generally believed that integrase tetramers catalyzed the integration of other retroviruses. The elucidation of new structures from four different retroviruses over the past year has however revealed this is not the case. The number of integrase molecules required to construct the conserved intasome core structure differs between viral species. While four subunits suffice for spumaviruses, α- and β-retroviruses require eight and the lentiviruses use up to sixteen. Herein we described these alternative architectures, highlighting both evolutionary and structural constraints that result in the different integrase-DNA stoichiometries across Retroviridae.

Figure 1

Retroviral integration and intasome nucleoprotein complexes. Shown are representative PFV intasome structures (SSC, pdb accession code 4e7h; CSC, pdb code 3oy9; TCC, pdb code 3os2; STC, pdb code 3os0). The intasomes consist of purified recombinant IN protein and synthetic oligonucleotides that model the U5 ends of viral DNA. Two DNA binding IN protomers are painted blue and green, whereas supportive IN molecules are cyan. Adjacent DNA end sequences are color-coded to match the transferred magenta and non-transferred orange viral DNA strands in the structures. Short vertical arrows, scissile phosphodiester bonds. Target DNA is shown in grey; during strand transfer, the DNA is cleaved with a 4 bp staggered cut.

Figure 2

Retroviral intasome structures and the CIC. The PFV structure is an underside view of the CSC relative to Figure 1, labeled for individual IN domains. The other intasome structures (MMTV CSC, pdb code 3jca; RSV STC, pdb code 5ejk; tetrameric HIV-1 STC, pdb code 5u1c; MVV CSC; pdb code 5m0q) are similarly oriented, size matched for common CIC components (colored). Synaptic CTDs that are donated from flanking IN dimers are shown in yellow. The portions of these structures that do not form part of the CIC are deemphasized by light gray. Viral DNA strands are black, and the target DNA strands from the RSV and HIV-1 STC structures, as well as the LEDGF/p75 IN-binding domain from the HIV-1 dodecamer, were omitted for clarity. Numbers indicate the number of IN molecules within each structure.