The SIV Surface Spike Imaged by Electron Tomography: One Leg or Three?

Strategies to inhibit the cellular entry of human immunodeficiency virus type 1 (HIV-1) constitute an important element of current approaches to develop an effective vaccine against AIDS [1]. The entry of viruses such as HIV-1, and its closely related simian counterpart simian immunodeficiency virus (SIV), is mediated by the interaction of glycoprotein spikes on the viral membrane surface with receptors on the target cell membrane. The viral spike itself is a trimeric complex of gp41, a membrane protein which anchors the spike to the membrane, and gp120 which is non-covalently associated with gp41. Cartoon representations of the spike as a trimeric entity abound, but what does it really look like? So far, structural information on the spike has come largely from X-ray crystallographic analysis of the core of the gp120 monomer in the CD4-liganded [2] or unliganded forms [3], and of trimers of the helical regions of gp41 in a post-fusion conformation [4]. The structure of an intact trimeric spike itself has, nevertheless, remained elusive.