Abstract
Inositol hexakisphosphate (IP6) potently stimulates HIV-1 particle assembly in vitro and infectious particle production in vivo. However, knockout cells lacking inositol-pentakisphosphate 2-kinase (IPPK-KO), the enzyme that produces IP6 by phosphorylation of inositol pentakisphosphate (IP5), were still able to produce infectious HIV-1 particles at a greatly reduced rate. HIV-1 in vitro assembly can also be stimulated to a lesser extent with IP5, but until recently, it was not known if IP5 could also function in promoting assembly in vivo. Here we addressed whether there is an absolute requirement for IP6 or IP5 in the production of infectious HIV-1 particles. IPPK-KO cells expressed no detectable IP6 but elevated IP5 levels and displayed a 20-100-fold reduction in infectious particle production, correlating with lost virus release. Transient transfection of an IPPK expression vector stimulated infectious particle production and release in IPPK-KO but not wildtype cells. Several attempts to make IP6/IP5 deficient stable cells were not successful, but transient expression of the enzyme multiple inositol polyphosphate phosphatase-1 (MINPP1) into IPPK-KOs resulted in near ablation of IP6 and IP5. Under these conditions, we found that HIV-1 infectious particle production and virus release were essentially abolished (1000-fold reduction) demonstrating an IP6/IP5 requirement. However, other retroviruses including a Gammaretrovirus, a Betaretrovirus, and two non-primate Lentiviruses displayed only a modest (3-fold) reduction in infectious particle production from IPPK-KOs and were not significantly altered by expression of IPPK or MINPP1. The only other retrovirus found to show a clear IP6/IP5 dependence was the primate (macaque) Lentivirus Simian Immunodeficiency Virus, which displayed similar sensitivity as HIV-1. We were not able to determine if producer cell IP6/IP5 is required at additional steps beyond assembly because viral particles devoid of both molecules could not be generated. Finally, we found that loss of IP6/IP5 in viral target cells had no effect on permissivity to HIV-1 infection.
Highlights
The HIV-1 structural protein Gag is produced in the cytoplasm and traffics to the plasma membrane where it assembles into a viral particle that buds from the host membrane [1]
We further show that the IP6 and IP5 requirement is a feature of primate lentiviruses, but not all retroviruses, and that IP6 or IP5 is required in the producer but not the target cell for HIV-1 infection
inositol-polyphosphate multikinase (IPMK) contributes to IP6 synthesis and infectious virus production
Summary
The HIV-1 structural protein Gag is produced in the cytoplasm and traffics to the plasma membrane where it assembles into a viral particle that buds from the host membrane [1]. The C-terminal CA and SP1 domains contain an alpha-helix that forms a sixhelix bundle with the other Gag proteins in the hexamer [4,5]. This bundle is important in formation and stabilization of the immature lattice [4,5]. During or shortly after budding from the cell, the viral protease cleaves the Gag polyprotein into its constitutive components, which separates CA from SP1 and eliminates the six-helix bundle [1,6]. The liberated CA protein assembles into a structurally distinct ’mature’ lattice, which forms the viral core [1,7]
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