Identification of a host protein essential for assembly of immature HIV-1 capsids

Abstract

To form an immature HIV-1 capsid, 1,500 HIV-1 Gag (p55) polypeptides must assemble properly along the host cell plasma membrane. Insect cells and many higher eukaryotic cell types support efficient capsid assembly, but yeast and murine cells do not, indicating that host machinery is required for immature HIV-1 capsid formation. Additionally, in a cell-free system that reconstitutes HIV-1 capsid formation, post-translational assembly events require ATP and a subcellular fraction, suggesting a requirement for a cellular ATP-binding protein. Here we identify such a protein (HP68), described previously as an RNase L inhibitor, and demonstrate that it associates post-translationally with HIV-1 Gag in a cell-free system and human T cells infected with HIV-1. Using a dominant negative mutant of HP68 in mammalian cells and depletion-reconstitution experiments in the cell-free system, we demonstrate that HP68 is essential for post-translational events in immature HIV-1 capsid assembly. Furthermore, in cells the HP68-Gag complex is associated with HIV-1 Vif, which is involved in virion morphogenesis and infectivity. These findings support a critical role for HP68 in post-translational events of HIV-1 assembly and reveal a previously unappreciated dimension of host-viral interaction.

The biological regulation of ABCE1

“…As a negative regulator of the 2-5A/RNase L system, it acts in regulation of RNA stability of cell, might be also involved in a broad range of biological functions including viral infection, tumor cell proliferation, and antiapoptosis (3,4). The protein was then demonstrated as a host factor essential for assembly of primate lentivirus capsids, especially for HIV-1 (5,6). Recent evidence indicated new important roles of ABCE1 protein in translation initiation, elongation, termination, and ribosome recycling in eukaryotes (7)(8)(9).…”

Rustless translation

“…Analysis of capsid assembly in cell free systems indicated an energy-dependent step and an important role of ABCE1. ABCE1 is found in assembly intermediates at the plasma membrane but not in mature capsids (Zimmerman et al , 2002 ;Lingappa et al , 2006 ). Hereby, the nucleocapsid (NC) domain of Gag binds viral genomic RNA and is also required for interaction with ABCE1, albeit indirectly (Klein et al , 2011 ).…”

“…RNase-L is an interferon-dependent polysome-associated antiviral defense enzyme that is activated by 2 ′ -5 ′ oligoadenylates and degrades mRNA and single-stranded viral RNA in virus infected cells (Silverman , 2007 ), but also has a variety of additional functions (Chakrabarti et al , 2011 ). In 2002, ABCE1 was found to be required for the assembly of immature HIV-1 capsids and it has been suggested that it accounts for the ATP dependence of the rather poorly understood capsid assembly process (Zimmerman et al , 2002 ). While these early results made ABCE1 an interesting target for biomedical research and therapeutic intervention, the cellular function of ABCE1 remained unclear.…”

“…Here, ABCE1 interacts with the Gag polypeptide, which is the only viral protein required to assemble and release virus like particles (Zimmerman et al , 2002 ;Dooher and Lingappa , 2004 ;Lingappa et al , 2006 ;Klein et al , 2011 ). After its synthesis from translation of full length viral genomic RNA, 3000 -5000 Gag polypeptides assemble in a stepwise process at the plasma membrane, forming buds that produce immature capsids (Briggs and Krausslich , 2011 ).…”

“…When I started my own laboratory in 2001, very little was known about this protein and its only function was a role in interfering with RNase-L activity (hence it is known as RNase-L inhibitor, Rli1) in the innate immune system (Bisbal et al , 1995 ). Shortly after, ABCE1 was found also as a 68 kDa host protein (named HP68 then) that was implicated in the assembly of immature capsids of human immunodeficiency virus (Zimmerman et al , 2002 ). However, ABCE1 is exceptionally conserved in evolution and must have a central, yet to be discovered function in the cell physiology of all eukaryotes and archaeal that goes beyond its relatively narrow role in RNase-L inhibition and HIV capsid assembly.…”