Dialysis purification of integrase-DNA complexes provides high-resolution atomic force microscopy images: dimeric recombinant HIV-1 integrase binding and specific looping on DNA

Going beyond Integration: The Emerging Role of HIV-1 Integrase in Virion Morphogenesis

The HIV-1 integrase enzyme (IN) plays a critical role in the viral life cycle by integrating the reverse-transcribed viral DNA into the host chromosome. This function of IN has been well studied, and the knowledge gained has informed the design of small molecule inhibitors that now form key components of antiretroviral therapy regimens. Recent discoveries unveiled that IN has an under-studied yet equally vital second function in human immunodeficiency virus type 1 (HIV-1) replication. This involves IN binding to the viral RNA genome in virions, which is necessary for proper virion maturation and morphogenesis. Inhibition of IN binding to the viral RNA genome results in mislocalization of the viral genome inside the virus particle, and its premature exposure and degradation in target cells. The roles of IN in integration and virion morphogenesis share a number of common elements, including interaction with viral nucleic acids and assembly of higher-order IN multimers. Herein we describe these two functions of IN within the context of the HIV-1 life cycle, how IN binding to the viral genome is coordinated by the major structural protein, Gag, and discuss the value of targeting the second role of IN in virion morphogenesis.

HIV-1 Integrase Binds the Viral RNA Genome and Is Essential during Virion Morphogenesis

While an essential role of HIV-1 integrase (IN) for integration of viral cDNA into human chromosome is established, studies with IN mutants and allosteric IN inhibitors (ALLINIs) have suggested that IN can also influence viral particle maturation. However, it has remained enigmatic as to how IN contributes to virion morphogenesis. Here, we demonstrate that IN directly binds the viral RNA genome in virions. These interactions have specificity, as IN exhibits distinct preference for select viral RNA structural elements. We show that IN substitutions that selectively impair its binding to viral RNA result in eccentric, non-infectious virions without affecting nucleocapsid-RNA interactions. Likewise, ALLINIs impair IN binding to viral RNA in virions of wild-type, but not escape mutant, virus. These results reveal an unexpected biological role of IN binding to the viral RNA genome during virion morphogenesis and elucidate the mode of action of ALLINIs.

HIV-1 integrase multimerization as a therapeutic target

Multimeric HIV-1 integrase (IN) plays an essential, multifunctional role in virus replication and serves as an important therapeutic target. Structural and biochemical studies have revealed the importance of the ordered interplay between IN molecules for its function. In the presence of viral DNA ends, individual IN subunits assemble into a tetramer and form a stable synaptic complex (SSC), which mediates integration of the reverse transcribed HIV-1 genome into chromatin. Cellular chromatin-associated protein LEDGF/p75 engages the IN tetramer in the SSC and directs HIV-1 integration into active genes. A mechanism to deregulate the productive interplay between IN subunits with small molecule inhibitors has recently received considerable attention. Most notably, allosteric IN inhibitors (ALLINIs) have been shown to bind to the IN dimer interface at the LEDGF/p75 binding pocket, stabilize interacting IN subunits, and promote aberrant, higher order IN multimerization. Consequently, these compounds impair formation of the SSC and associated LEDGF/p75-independent IN catalytic activities as well as inhibit LEDGF/p75 binding to the SSC in vitro. However, in infected cells, ALLINIs more potently impaired correct maturation of virus particles than the integration step. ALLINI treatments induced aberrant, higher order IN multimerization in virions and resulted in eccentric, non-infectious virus particles. These studies have suggested that the correctly ordered IN structure is important for virus particle morphogenesis and highlighted IN multimerization as a plausible therapeutic target for developing new inhibitors to enhance treatment options for HIV-1-infected patients.