HIV-1 Matrix Protein Interactions with tRNA: Implications for Membrane Targeting

Abstract

The N-terminally myristoylated matrix (MA) domain of the HIV-1 Gag polyprotein promotes virus assembly by targeting Gag to the inner leaflet of the plasma membrane. Recent studies indicate that, prior to membrane binding, MA associates with cytoplasmic tRNAs (including tRNA), and in vitro studies of tRNA-dependent MA interactions with model membranes have led to proposals that competitive tRNA interactions contribute to membrane discrimination. We have characterized interactions between native, mutant, and unmyristylated (myr-) MA proteins and recombinant tRNA by NMR spectroscopy and isothermal titration calorimetry. NMR experiments confirm that tRNA interacts with a patch of basic residues that are also important for binding to the plasma membrane marker, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P]. Unexpectedly, the affinity of MA for tRNA (K = 0.63 ± 0.03 μM) is approximately 1 order of magnitude greater than its affinity for PI(4,5)P-enriched liposomes (K = 10.2 ± 2.1 μM), and NMR studies indicate that tRNA binding blocks MA association with liposomes, including those enriched with PI(4,5)P, phosphatidylserine, and cholesterol. However, the affinity of MA for tRNA is diminished by mutations or sample conditions that promote myristate exposure. Since Gag-Gag interactions are known to promote myristate exposure, our findings support virus assembly models in which membrane targeting and genome binding are mechanistically coupled.

Nucleic acid–induced dimerization of HIV-1 Gag protein

“…10. In this way, nucleic acid-induced dimerization observed here ties in with the previous observation that myristate exposure in MA, which targets MA to the membrane, is linked to MA and CA interactions (50) and related to Gag nucleic acid binding (52).…”

“…6). This is consistent with the assembly model where not only NC but initially also the MA domain binds to NA (3,12,(46)(47)(48)(49)(50)(51). It is thought that simultaneous binding of both domains to RNA may be important for correct intracellular targeting, in a mechanism where engagement of MA with its preferred ligand phosphatidylinositol 4,5bisphosphate at the plasma membrane is concurrent with conformational transitions of Gag promoting assembly (47,52,53).…”

Maturation of the matrix and viral membrane of HIV-1

“…This mode of PI(4,5)P2 binding, in which the 2´ acyl chain of the lipid would have to be pulled out of the bilayer, is no longer widely considered relevant to MA-membrane interactions during virus assembly, which instead are likely to be between the PI(4,5)P2 headgroup and the membrane-facing basic MA residues (13)(14)(15). MA interacts with nucleic acids in the cytosol, in particular tRNA, via the HBR (16,17). These observations support a model in which exchange of nucleic acid for the PI(4,5)P2 head group promotes myristate exposure and stabilization of PM binding during virus assembly (16,17).…”

Molecular determinants of Arc oligomerization and formation of virus-like capsids

“…The MA domain is critical for targeting of Gag to the plasma membrane, and this depends on MA myristoylation and the binding of host RNA. Specifically, binding of transfer RNAs to a basic region in MA is thought to block Gag interaction with internal membranes prior to reaching the highly acidic inner leaflet of the plasma membrane (Gaines et al, 2018). The Arc NTD has predicted homology and several functional similarities to retroviral MA (Campillos et al, 2006).…”