Review
Molecular Determinants that Regulate Plasma Membrane Association of HIV-1 Gag

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Abstract

Human immunodeficiency virus type 1 assembly is a multistep process that occurs at the plasma membrane (PM). Targeting and binding of Gag to the PM are the first steps in this assembly process and are mediated by the matrix domain of Gag. This review highlights our current knowledge on viral and cellular determinants that affect specific interactions between Gag and the PM. We will discuss potential mechanisms by which the matrix domain might integrate three regulatory components, myristate, phosphatidylinositol-(4,5)-bisphosphate, and RNA, to ensure that human immunodeficiency virus type 1 assembly occurs at the PM.

Section snippets

Gag Membrane Binding Is Driven by Bipartite Signal in MA

MA is the membrane proximal domain of Pr55Gag. The first 104 amino acids of the MA form a compact globular domain consisting of five major α-helices that are capped by a mixed three-stranded β-sheet.15, 16 MA is myristoylated at the amino-terminus and facilitates membrane binding of Gag.17, 18 In addition, a highly basic region (HBR) spanning residues 14–31 (Fig. 2a) is important for efficient membrane binding and proper targeting of Gag to the PM. These basic residues are clustered around the

The N-Terminal Myristoyl Moiety Is Sequestered in the Globular Domain of MA

Myristoylation is a process that occurs cotranslationally, where a 14-carbon saturated fatty acid, myristic acid, is attached to the N-terminal glycine that is exposed after the first methionine is removed. The enzyme that catalyzes this reaction is the myristoyl-coenzyme A:protein N-myristoyltransferase (for a detailed review, see Ref. 25). Mutating the N-terminal glycine to alanine severely reduces membrane binding of Gag and inhibits virus particle release.10, 17, 18, 22, 26 In addition,

The MA HBR Binds Acidic Lipids

Although myristoylation is necessary, it is thought to be insufficient for efficient membrane binding of proteins.42, 43 Myristoylation only provides reversible membrane binding, and a second signal is thought to be required for strong localization of the myristoylated protein to the membrane.43 This second signal can be a polybasic cluster, a second acylation such as palmitoylation, or a protein–protein interaction that increases avidity of the protein complex. In the case of HIV-1 Gag, the

Interaction between the MA HBR and PI(4,5)P2 Is Important for PM Targeting and Efficient Membrane Binding of Gag

As alluded to earlier, recent studies have shown that the basic residues in the HBR interact with the acidic phospholipid, PI(4,5)P2.21, 38, 55, 56 PI(4,5)P2 belongs to a family of phospholipids called phosphoinositides. These lipids are derivatives of phosphatidylinositol and have a hydrophobic diacylglycerol backbone esterified to a polar inositol head group that can be phosphorylated at three of the five hydroxyl residues (Fig. 2b). Thus, seven different phosphatidylinositol phosphates

RNA Bound to the MA HBR Inhibits PI(4,5)P2-Independent Membrane Binding of Gag In Vitro

In addition to binding acidic lipids, several in vitro studies have shown that the MA HBR can bind RNA.56, 77, 78, 79, 80, 81, 82, 83 We hypothesize that RNA binding to MA could be a factor that necessitates the involvement of PI(4,5)P2 for efficient membrane binding of Gag. We have recently shown that RNase treatment of Gag synthesized in vitro using rabbit reticulocyte lysate significantly increases binding of Gag to control liposomes that lack PI(4,5)P2 but still contain another acidic

MA–PI(4,5)P2 Binding May Be Important for Coordinating Gag Membrane Binding with Other Assembly and Post-Assembly Events

As mentioned earlier, both myristate and HBR are necessary for efficient membrane binding of Gag. Either one alone is not sufficient in the context of full-length Gag in cells, as supported by observations that both non-myristoylated Gag and Gag derivatives with substitutions of all basic residues in the HBR do not bind membranes efficiently.10, 17, 19, 21, 26 However, in contrast to amino acid substitutions in the HBR, deletion of the entire MA globular head except for the myristoylation

Conclusions

Binding of Gag to the PM is the essential early step in HIV-1 assembly. These processes are likely modulated by MA in three different but coordinated ways: positively by interacting with PI(4,5)P2, negatively by suppressing myristate-mediated hydrophobic interaction, and negatively by interacting with RNA. With the present knowledge on Gag membrane binding, several types of drugs can be developed for inhibiting this process during HIV-1 assembly. Blocking N-terminal myristoylation of Gag or

Acknowledgements

We thank members of our laboratory for helpful discussions and critical review of the manuscript and S. Meshinchi for the help in electron microscopy. Our work related to the topics discussed in this review is supported by the National Institute of Allergy and Infectious Diseases (R01 AI071727 and R56 AI089282), American Heart Association (0850133Z), and amfAR (107449-45-RGHF).

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