Molecular Interactions

Human immunodeficiency virus type 1 is an enveloped retrovirus that infects CD4+ T lymphocytes [1,2]. T lymphocytes are mobile and, once infected, live only a few days [3]. Thus, HIV must not only find the proper cell, but it must do so repeatedly over the hundreds of cycles of lymphocyte turnover that typify its persistent infection. The combined function of finding host cells and of properly initiating the viral fusion machinery is accomplished by the HIV-1 gp120 exterior envelope glycoprotein (reviewed by Wyatt and Sodroski [4]). The gp120 glycoprotein is initially synthesized as part of a trimeric gp160 glycoprotein, which is cleaved by cellular proteases into gp120 (N-terminal portion, roughly 500 amino acids, highly glycosylated) and gp41 (C-terminal portion, roughly 350 amino acids, transmembrane spanning) components. Noncovalent interactions keep this trimer of het-erodimers associated as the biologically active viral spike.

The gp120 glycoprotein binds to the N-terminal membrane distal domain of the cellular CD4 receptor [5-8]. This interaction triggers conformational changes in gp120 that induce the formation of a binding site for the coreceptor, a member of the chemokine receptor family, either CCR5 or CXCR4 [9,10]. Binding by coreceptor initiates additional conformational changes that trigger the gp41 fusion machinery, leading to a fusion of the viral and cellular membranes and entry of the HIV-1 genome into the host cytoplasm (Fig. 1).

Although the small size of the virion (approximately 1000 A in diameter) enhances diffusion, HIV virions are cleared rapidly from serum, and HIV gp120 employs several means to enhance receptor encounters. First, nonspecific electrostatic interactions generate binding to cell-surface polyanions such as heparin sulfate [11,12]. This electrostatic adhesion allows two-dimensional cell-surface scanning, enhancing the probability of gp120/cell-surface CD4 encounters. Second, it abducts innate immune responses on dendritic cells to promote infection in trans [13]. The gp120 glycopro-tein displays high mannose N-linked glycans that bind to DC-SIGN and other dendritic cell receptors [14]. These receptors are used in innate immunity to scavenge for microbial invaders and to activate immune recognition, but binding to HIV gp120 results in the efficient presentation of the virus to suitable target cells (reviewed by Pohlmann et al. [15]).

These molecular interactions highlight several unique features of viral interactions. First, the ingenious manner by which the virus usurps host systems, with a redundancy of mechanisms to ensure viral propagation. Second, virions are not meta-bolically active, which has several diverse implications: highly specific recognition must occur without metabolic activation or proofreading; viral motion is propelled solely by Brownian forces; and large thermodynamic barriers (such as membrane fusion) must be overcome by using only energy stored in folded proteins. Third, HIV viral proteins function under severe constraints on genome size. The entire HIV genome is only 10 kilobase-pairs. These genome constraints are reflected at the DNA level by overlapping reading frames. On the protein level, they lead to a condensed multifunctionality. While eukaryotic recognition often involves a number of different proteins, each performing a specific task, the entire HIV recognition and entry procedure is accomplished with only two proteins. Multiple functionalities are encoded by different subunits as well as by different conformational states of the same polypeptide (Fig. 1).

Coiled Coil Refolding

Figure 1 Molecular interactions and conformational states of the HIV-1 envelope glycoproteins. The top panel of figures illustrates the molecular interactions of the HIV-1 envelope glycoproteins. In the leftmost figure, a schematic of the biologically active viral spike is depicted, with gp120 molecules attached to the gp41 ectodomain. The subsequent figures diagram binding of CD4, followed by co-receptor (gray ovals), which initiates the gp41 fusion machinery: the N-terminal fusion peptide of gp41 is thrown into the target cell, and dramatic refolding of gp41 results in a final coiled-coil structure, with gp41 N- and C-ectodomain termini proximal. (For clarity, only the gp41 ectodomain is depicted. Thus the gp41 "C" corresponds to the membrane proximal portion of the gp41 ectodomain.) The boxed panel of figures illustrates these changes in the context of a single gp120 protomer. The leftmost figure shows the quiescent gp120. Basic surfaces (++) and high mannose N-linked glycan (o o) enhance cell-surface attachment and presentation to CD4+ lymphocytes. In this quiescent state, the CD4 binding site is occluded by the V1/V2 variable loop, and the co-receptor binding site is not formed. Upon binding to CD4 (second figure), the inner and outer domains reorganize, forming both the Phe-43 cavity (at the center of gp120) and the bridging sheet and partially destabilizing quaternary interactions. Chemokine receptor binding (third figure) to the newly formed bridging sheet and V3 loop (light gray) trigger the gp41 fusion machinery. (Boxed panel adapted from Kwong, P. D. et al., Nature, 393, 648-659, 1998, Fig. 5.)

Figure 1 Molecular interactions and conformational states of the HIV-1 envelope glycoproteins. The top panel of figures illustrates the molecular interactions of the HIV-1 envelope glycoproteins. In the leftmost figure, a schematic of the biologically active viral spike is depicted, with gp120 molecules attached to the gp41 ectodomain. The subsequent figures diagram binding of CD4, followed by co-receptor (gray ovals), which initiates the gp41 fusion machinery: the N-terminal fusion peptide of gp41 is thrown into the target cell, and dramatic refolding of gp41 results in a final coiled-coil structure, with gp41 N- and C-ectodomain termini proximal. (For clarity, only the gp41 ectodomain is depicted. Thus the gp41 "C" corresponds to the membrane proximal portion of the gp41 ectodomain.) The boxed panel of figures illustrates these changes in the context of a single gp120 protomer. The leftmost figure shows the quiescent gp120. Basic surfaces (++) and high mannose N-linked glycan (o o) enhance cell-surface attachment and presentation to CD4+ lymphocytes. In this quiescent state, the CD4 binding site is occluded by the V1/V2 variable loop, and the co-receptor binding site is not formed. Upon binding to CD4 (second figure), the inner and outer domains reorganize, forming both the Phe-43 cavity (at the center of gp120) and the bridging sheet and partially destabilizing quaternary interactions. Chemokine receptor binding (third figure) to the newly formed bridging sheet and V3 loop (light gray) trigger the gp41 fusion machinery. (Boxed panel adapted from Kwong, P. D. et al., Nature, 393, 648-659, 1998, Fig. 5.)

Diabetes Sustenance

Diabetes Sustenance

Get All The Support And Guidance You Need To Be A Success At Dealing With Diabetes The Healthy Way. This Book Is One Of The Most Valuable Resources In The World When It Comes To Learning How Nutritional Supplements Can Control Sugar Levels.

Get My Free Ebook


Post a comment