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Membrane Binding of HIV-1 Matrix Protein: Dependence on Bilayer Composition and Protein Lipidation

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Membrane Binding of HIV-1 Matrix Protein: Dependence on Bilayer Composition and Protein Lipidation crossmark Membrane Binding of HIV-1 Matrix Protein: Dependence on Bilayer Composition and Protein Lipidation Marilia Barros,a Frank Heinrich,a,c Siddhartha A. K. Datta,d Alan Rein,d Ioannis Karageorgos,e,f Hirsh Nanda,a,c* Mathias Löschea,b,c Department of Physicsa and Department of Biomedical Engineeringb, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA; NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland, USAc; HIV Dynamics and Replication Program, Center for Cancer Research, National Institute of Cancer, Frederick, Maryland, USAd; Biomolecular Measurement Division, National Institute of Standards and Technology, Gaithersburg, Maryland, USAe; Institute for Downloaded from Bioscience and Biotechnology Research, Rockville, Maryland, USAf ABSTRACT By assembling in a protein lattice on the host’s plasma membrane, the retroviral Gag polyprotein triggers formation of the viral protein/membrane shell. The MA domain of Gag employs multiple signals—electrostatic, hydrophobic, and lipid-specific—to bring the protein to the plasma membrane, thereby complementing protein-protein interactions, located in full-length Gag, in lattice formation. We report the interaction of myristoylated and unmyristoylated HIV-1 Gag MA domains with bilayers com- posed of purified lipid components to dissect these complex membrane signals and quantify their contributions to the overall http://jvi.asm.org/ interaction. Surface plasmon resonance on well-defined planar membrane models is used to quantify binding affinities and amounts of protein and yields free binding energy contributions, ⌬G, of the various signals. Charge-charge interactions in the absence of the phosphatidylinositide PI(4,5)P2 attract the protein to acidic membrane surfaces, and myristoylation increases the affinity by a factor of 10; thus, our data do not provide evidence for a PI(4,5)P2 trigger of myristate exposure. Lipid-specific inter- actions with PI(4,5)P2, the major signal lipid in the inner plasma membrane, increase membrane attraction at a level similar to that of protein lipidation. While cholesterol does not directly engage in interactions, it augments protein affinity strongly by fa- cilitating efficient myristate insertion and PI(4,5)P2 binding. We thus observe that the isolated MA protein, in the absence of on August 3, 2016 by NIST RESEARCH LIBRARY protein-protein interaction conferred by the full-length Gag, binds the membrane with submicromolar affinities. IMPORTANCE Like other retroviral species, the Gag polyprotein of HIV-1 contains three major domains: the N-terminal, myristoylated MA domain that targets the protein to the plasma membrane of the host; a central capsid-forming domain; and the C-terminal, ge- nome-binding nucleocapsid domain. These domains act in concert to condense Gag into a membrane-bounded protein lattice that recruits genomic RNA into the virus and forms the shell of a budding immature viral capsid. In binding studies of HIV-1 Gag MA to model membranes with well-controlled lipid composition, we dissect the multiple interactions of the MA domain with its target membrane. This results in a detailed understanding of the thermodynamic aspects that determine membrane as- sociation, preferential lipid recruitment to the viral shell, and those aspects of Gag assembly into the membrane-bound protein lattice that are determined by MA. he polyprotein Gag is an essential component for the repro- proteins. They also differ in cotranslational modifications, since Tduction of retroviruses, such as HIV-1, in infected cells. In the some, such as HIV-1 MA, are lipidated whereas those of others, production of new viruses, many copies of Gag assemble laterally, such as Rous sarcoma virus (RSV), are not. Both the myristoyl- either in the cytoplasm or on the plasma membrane (PM) of the ation of HIV MA and its basic patch are required for correct PM host, thus acquiring their lipid envelope as they bud from the cell. targeting (4, 5). Indeed, Gag protein that included an MA-Src Distinct retroviral genera encode Gag proteins which all show a chimera in which the first 31 MA residues were replaced by the similar domain architecture, with an N-terminal matrix (MA) N-terminal end of Src was found to bind membranes and form domain that targets the PM, followed by the capsid (CA) and nucleocapsid (NC) proteins that are connected by minor domains or unstructured peptide linkers. Across retroviral genera, MA do- Received 5 November 2015 Accepted 15 February 2016 mains also share a high degree of structural homology (1). Most of Accepted manuscript posted online 24 February 2016 their N-terminal portions comprise five or six ␣-helices and adopt Citation Barros M, Heinrich F, Datta SAK, Rein A, Karageorgos I, Nanda H, Lösche a compact globular fold, whereas their C termini are often flexible M. 2016. Membrane binding of HIV-1 matrix protein: dependence on bilayer or unstructured (2). Moreover, retroviral MA domains share elec- composition and protein lipidation. J Virol 90:4544–4555. doi:10.1128/JVI.02820-15. trostatic homology: basic amino acids are clustered on one surface Editor: W. I. Sundquist of their N-terminal “globular heads,” resulting in a basic patch Address correspondence to Alan Rein, [email protected], or that interacts electrostatically with the PM of the infected cell (3). Mathias Lösche, [email protected]. On the other hand, the MA domains of different retroviruses dif- * Present address: Hirsh Nanda, Janssen, LLC, Spring House, Pennsylvania, USA. fer in their net charges in this basic patch, which suggests differing Copyright © 2016, American Society for Microbiology. All Rights Reserved. contribution of electrostatic interactions in the targeting of these 4544 jvi.asm.org Journal of Virology May 2016 Volume 90 Number 9 Components of HIV-1 Gag Matrix Membrane Binding Downloaded from http://jvi.asm.org/ FIG 1 Schematic representation of the experimental configuration of the SPR measurements on an stBLM. The ultrathin gold layer on a glass substrate is passivated by a densely packed mercaptoethanol layer on which the membrane is grafted. The surface-ligated bilayer is highly hydrated at both interfaces and in-plane fluid, and yet resilient to experimental manipulation, and virtually defect-free. Its lipidic composition can be precisely controlled. on August 3, 2016 by NIST RESEARCH LIBRARY virus-like particles with the same characteristic dependence on gets of HIV (22, 23), show similar membrane compositions (15) basic residues and myristoylation as native HIV Gag (4). Although even though the global compositions of macrophage and T-cell only MA is in direct contact with the surrounding membrane in membranes are different. This suggests that HIV-1 recruits spe- the completed viral shell, the remaining domains of the Gag poly- cific membrane components and thus creates a favorable lipidic protein contribute to Gag membrane affinity. It is well docu- environment for its assembly (23). mented that CA-CA interactions drive Gag oligomerization (6, 7), The binding of HIV-1 MA to bilayer membranes has already and artificial multimerization of MA enhances its affinity to mem- been extensively studied (8, 24, 25). MA interactions with branes by an order of magnitude (8). Moreover, a recent study PI(4,5)P2 not only direct Gag assembly to specific regions of the identifies protein-protein contacts in the SPA segment of RSV Gag PM but may also help trigger myristate exposure from its seques- that stabilize oligomers and increase membrane affinity, leading in tered state within the protein (26). Entropic effects may contrib- turn to pronounced cooperativity in membrane binding (9). ute, since it has been reported that myristate exposure favors MA In HIV, Gag is synthesized in the cytoplasm, migrates to the trimerization (27, 28). Moreover, some evidence suggests that the cellular periphery, and eventually targets the surface of the PM, unsaturated chain of PI(4,5)P2 binds MA upon myristate mem- where the assembly of immature virus occurs (10). As in other brane insertion (26, 28), which would enhance Gag affinity to lipid retroviral genera, the MA domain of HIV-1 Gag is the structural rafts. On the other hand, important questions remain poorly un- motif that mediates targeting to the PM (11) where lateral assem- derstood. For example, a clear delineation of how individual bi- bly of the viral shell occurs. Distinct molecular signals comple- layer components contribute to MA binding has not yet been ment each other in positioning the protein in an orientation pro- achieved, and in particular the impact of protein myristoylation ductive for assembly on the correct target membrane (12): on protein-membrane interactions has not been quantitatively electrostatic interactions between the basic patch of residues on assessed. the globular head and anionic membrane lipids, hydrophobic in- In this study, we use surface plasmon resonance (SPR) spec- teraction between MA’s myristoylated N terminus and the mem- troscopy on synthetic solid-supported bilayers, referred to as brane, and specific binding of the protein to phosphatidylinositol- sparsely tethered bilayer lipid membranes (stBLMs), to parse pro- 4,5-bisphosphate [PI(4,5)P2](13), a low-abundance phospholipid tein-membrane interactions into their components. In combina- found principally in the cytoplasmic leaflet of the PM (14). Lip- tion, SPR performed on stBLMs offers unique advantages for the idomic studies show that raft-related lipids, such as sphingomy- study of viral assembly processes on membranes. Both the stBLM elin and cholesterol, are enriched in the HIV-1 envelope (15–18). model system (29) and the SPR method for studying intermolec- This is consistent with the hypothesis that HIV-1 assembles on ular interactions (30) are well established. SPR offers information microdomains in the PM, possibly lipid rafts, with specific lipid not only on the affinity of a ligand to its target surface but also on compositions (19–21). Moreover, virion membranes budded the equilibrium load of the ligand at that surface (30), i.e., the from macrophages and from T cells, two distinct preferential tar- protein density on the membrane surface.
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