A Sticky Spike
HIV infects macrophages and T cells when the major protein “spike” on its surface binds to CD4 and a chemokine coreceptor (CCR5 or CXCR4) on the immune cells. This spike contains a trimer of glycoprotein 120 (gp120) sitting atop trimer of gp41 embedded in the viral membrane. gp120 and gp41 are encoded by a single viral gene (Envelop), with its resulting polypeptide cleaved by the host protease Furin. Gp120 binds directly to CD4, and gp41 facilitates membrane fusion.
Image: On left, 3D structure of a single simian immunodeficiency virus (SIV) obtained with cryo-electron tomography; the architecture of SIV’s surface ‘spikes’ (blue) is similar to that of HIV. On right, 3D cryo-electron tomography reveals HIV-1’s glycoprotein “spike” in complex with a soluble CD4 protein and a coreceptor mimic (17b) at ~20 Å resolution. Three copies of the coordinates for the ternary complex between gp120 (red), soluble CD4 (yellow) and 17b (cyan) have been fitted to the density map to produce a molecular model for spike structure. Learn more in White et al (2010) and Liu et al. (2008).