Architecture and Assembly of HIV Integrase Multimers in the Absence of DNA Substrates*

Background: No full-length structure of HIV integrase alone has been reported. Results: We elucidated the architectures of dimers and tetramers of full-length HIV-1 integrase in solution. Conclusion: HIV apo-integrase can assemble in two alternate dimer forms: a reaching and a core-core dimer. The tetramer comprises two stacked reaching dimers, stabilized by core-core interactions. Significance: New insights into HIV integrase architecture and its inhibition are suggested. We have applied small angle x-ray scattering and protein cross-linking coupled with mass spectrometry to determine the architectures of full-length HIV integrase (IN) dimers in solution. By blocking interactions that stabilize either a core-core domain interface or N-terminal domain intermolecular contacts, we show that full-length HIV IN can form two dimer types. One is an expected dimer, characterized by interactions between two catalytic core domains. The other dimer is stabilized by interactions of the N-terminal domain of one monomer with the C-terminal domain and catalytic core domain of the second monomer as well as direct interactions between the two C-terminal domains. This organization is similar to the “reaching dimer” previously described for wild type ASV apoIN and resembles the inner, substrate binding dimer in the crystal structure of the PFV intasome. Results from our small angle x-ray scattering and modeling studies indicate that in the absence of its DNA substrate, the HIV IN tetramer assembles as two stacked reaching dimers that are stabilized by core-core interactions. These models of full-length HIV IN provide new insight into multimer assembly and suggest additional approaches for enzyme inhibition.