Membrane interaction of retroviral Gag proteins

Abstract

Assembly of an infectious retroviral particle relies on multimerization of the Gag polyprotein at the inner leaflet of the plasma membrane. The three domains of Gag common to all retroviruses - MA, CA, and NC - provide the signals for membrane binding, assembly, and viral RNA packaging, respectively. These signals do not function independently of one another. For example, Gag multimerization enhances membrane binding and is more efficient when NC is interacting with RNA. MA binding to the plasma membrane is governed by several principles, including electrostatics, recognition of specific lipid head groups, hydrophobic interactions, and membrane order. HIV-1 uses many of these principles while Rous sarcoma virus (RSV) appears to use fewer. This review describes the principles that govern Gag interactions with membranes, focusing on RSV and HIV-1 Gag. The review also defines lipid and membrane behavior, and discusses the complexities in determining how lipid and membrane behavior impact Gag membrane binding.

Keywords: HIV-1; RSV; assembly; lipid; liquid ordered; plasma membrane.

FIGURE 1

Model membrane phase behavior and lipid parameters. (A) Color merged GUV’s of simultaneously collected fluorescence emission from C20:0-DiI and (16:0, Bodipy)-PC reconstructed from confocal microscopy z-scans (Zhao et al., 2007). The red emitting C20:0-DiI segregates to both the L_o and L_β membrane phase, and the green emitting (16:0, Bodipy)-PC segregates to the L_d membrane phase. The lipid composition of each GUV is denoted by a black circle and a gray square on the phase diagram (B). (B) The phase diagram is adapted from Zhao et al. (2007), Heberle et al. (2010) and Konyakhina et al. (2013). The vertices of the triangle represent pure components: 100% cholesterol, 100% low Tm lipid dioleoyl-PC (DOPC), and 100% high Tm lipid distearoyl-PS (DSPC). Each side of the triangle represents a binary mixture. The left side of the triangle represents lipid mixtures resulting in L_d membranes, and the right side of the triangle represents lipid mixtures resulting in L_o membranes. As the low Tm lipid DOPC is replaced with the high Tm DSPC at a fixed cholesterol concentration, for example, above 42% the membrane phase becomes increasingly ordered as depicted by the green to red gradient trapezoid. The one phase region falls outside of the labeled two and three phase regions. Lipid mixtures that fall in the region of the phase diagram labeled with more than one phase, for example the black circle in the L_o + L_d region, result in membranes with co-existing phases. The dotted line through the black circle represents a tie line. The distance of the black circle from the phase boundaries along the tie line represents the percent of membrane that is L_d and L_o, approximately 25 and 75%, respectively. The intersection of the tie line with the left or right boundary denotes the composition of the respective membrane phase. In this example, the intersection of the tie line on the left side of the boundary corresponds to an L_d membrane with an approximate composition DOPC/DSPC/Chol (60/10/30). The intersection of the tie line on the right hand side of the boundary corresponds to an L_o membrane with an approximate composition DOPC/DSPC/Chol (20/40/40). When preparing GUVs with a charged lipid such as PS, it seems reasonable to exchange a percentage of the PC lipid for PS lipid. Using the black circle as an example, if one replaces all the DOPC (30%) with DOPS (30%), the resulting GUV would have an L_d region with 60% PS and an L_o region with 20% PS. This simplified example of exchanging PC for PS does not take into account the possible effect that lipid head group has on membrane phase behavior. (C) Molecular dynamics snapshot of DOPC/DSPC/Chol lipid segregation in a membrane bilayer. Line tension drives the formation of macro domains, due in part to differences in membrane thickness; L_o is thicker than L_d. Because the interface is costs energy, in some lipid mixtures, micro domains coalesce into macro domains to minimize the interface. (D) Properties of different membrane phases and acyl chain order (adapted from van Meer et al., 2008).