Lipids and membrane microdomains in HIV-1 replication

Abstract

Several critical steps in the replication cycle of human immunodeficiency virus type 1 (HIV-1) - entry, assembly and budding - are complex processes that take place at the plasma membrane of the host cell. A growing body of data indicates that these early and late steps in HIV-1 replication take place in specialized plasma membrane microdomains, and that many of the viral and cellular components required for entry, assembly, and budding are concentrated in these microdomains. In particular, a number of studies have shown that cholesterol- and sphingolipid-enriched microdomains known as lipid rafts play important roles in multiple steps in the virus replication cycle. In this review, we provide an overview of what is currently known about the involvement of lipids and membrane microdomains in HIV-1 replication.

Fig. 1

A simplified model of lipid-raft structure in biological membranes. The sphingolipids (violet) are enriched in the outer leaflet of the bilayer, whereas cholesterol (yellow) and phospholipids (brown and blue) are distributed in both leaflets. Note that lipids in the raft domain usually have long and saturated acyl chains (violet and brown), whereas the lipids excluded from these domains have shorter and unsaturated acyl chains (blue). GPI-anchored (orange) and dually-acylated proteins (green) are concentrated in raft domains. Transmembrane proteins (blue) and prenylated proteins (pink) generally reside in non-raft regions, whereas some transmembrane proteins (pink) associate with raft domains. Panel A depicts a lipid raft in cross section; B shows a top view, looking down on the membrane. In panel B, the raft boundary is represented by a dashed oval.

Fig. 1

A simplified model of lipid-raft structure in biological membranes. The sphingolipids (violet) are enriched in the outer leaflet of the bilayer, whereas cholesterol (yellow) and phospholipids (brown and blue) are distributed in both leaflets. Note that lipids in the raft domain usually have long and saturated acyl chains (violet and brown), whereas the lipids excluded from these domains have shorter and unsaturated acyl chains (blue). GPI-anchored (orange) and dually-acylated proteins (green) are concentrated in raft domains. Transmembrane proteins (blue) and prenylated proteins (pink) generally reside in non-raft regions, whereas some transmembrane proteins (pink) associate with raft domains. Panel A depicts a lipid raft in cross section; B shows a top view, looking down on the membrane. In panel B, the raft boundary is represented by a dashed oval.

Fig. 2

Schematic representation of HIV-1 Env proteins. The HIV-1 Env glycoprotein precursor (gp160) is proteolytically processed (arrow) into the surface glycoprotein gp120 and the transmembrane glycoprotein gp41. gp41 is composed of an ectodomain containing the fusion peptide (FP) and heptad repeat 1 and 2 (HR1 and HR2); a membrane-spanning domain (MSD); and cytoplasmic tail (CT).

Fig. 3

HIV-1 entry. Gp120 on the surface of the HIV-1 virion binds to CD4 and coreceptor (CCR5 and CXCR4) in lipid rafts (red) of the plasma membrane. Membrane fusion between viral and plasma membrane, mediated by the transmembrane Env glycoprotein gp41, results in viral entry and uncoating. Modified and reprinted with permission from Ono and Freed, 2005, (Ono and Freed, 2005). Copyright 2005, Elsevier Inc.

Fig. 4

Schematic representation of HIV-1 Gag proteins. The major domains in the Gag precursor (Pr55^Gag) - MA, CA, NC and p6 - are indicated. The N-terminal myristate (Myr) and the MA highly basic domain (+++) are shown.

Fig. 5

Schematic representation of HIV-1 assembly and release at the plasma membrane. Pr55^Gag is directed post-synthesis to the plasma membrane where it associates with cholesterol- and sphingolipid-rich lipid raft microdomains (red). Gag-Gag interactions lead to the assembly of viral particles resulting in highly multimerized Gag complexes that bud from the plasma membrane. The viral Env glycoproteins are incorporated during the assembly process and virions ultimately pinch off from the cell surface. Reprinted with permission from Waheed et al., 2009, (Waheed et al., 2009). HIV protocols 2^nd edition, copyright Humana press.

Fig. 6

HIV-1 Gag proteins copatch with the raft marker GM1 but not with the non-raft marker transferrin receptor (TfR). Jurkat cells expressing HIV-1 Gag were labeled with cholera toxin B subunit to stain GM1 or with anti-TfR antibody before fixation. Cells were then fixed and permeabilized, and incubated with anti-p17 MA antibody to detect Gag. Images collected in the z axis using a confocal microscope were projected in 3D volume with Imaris software. Merged images of Gag and GM1 show colocalization, whereas Gag and TfR are not colocalized. Reprinted with permission from Ono and Freed, 2005 (Ono and Freed, 2005). Copyright 2005, Elsevier Inc.

Fig. 7

Binding of the MA domain of HIV-1 Gag to PI(4,5)P₂. A. The N-terminal myristate (dark green) is sequestered in the globular core of MA. The MA highly basic region (blue patch), CA (red), NC and p6 (dark blue) are shown. B. Upon binding of MA to PI(4,5)P₂ in the inner leaflet of the lipid bilayer, the myristic acid (dark green) flips out from MA into the exposed conformation and inserts into the lipid bilayer. According to this model, the 2′-acyl chain (purple) of PI(4,5)P₂ is extruded from the membrane and docks into a groove in MA. C. Structure of PI(4,5)P₂. The two phosphates (red) are attached to the inositol ring (yellow) at position 4 and 5. The 2′-acyl chain (violet) is unsaturated and kinked and the 1′-acyl chain is saturated. Reprinted with permission from Freed, 2006, (Freed, 2006). Copyright 2006, The National Academy of Sciences of the USA. Based on the data of (Saad et al., 2006).

Fig. 8

HIV-1 Gag and Env concentrate in a raft-rich domain at the virological synapse. HIV-1-infected Jurkat effector cells (bottom) were incubated with primary CD4⁺ T cells (top) for 1 h at 37°C during which the cells were stained for Env (red), fixed and stained for the raft marker GM1 with cholera toxin B subunit (green). Cells were permeabilized and labeled with anti-p17/p24 antibody (blue). Merging of three colors shows colocalization, which is superimposed in white on the corresponding Nomarski image. Reprinted with permission from Jolly and Sattentau 2005, (Jolly and Sattentau, 2005). Copyright 2005, The American Society for Microbiology.