The second step of virus entry and the first absolutely required for infection entails binding of Env to its primary receptor, the host protein CD4 ( Maddon et al. This initiates the membrane fusion process as the fusion peptide of gp41 inserts into the target membrane, followed by six-helix bundle formation and complete membrane fusion (4). This causes conformational changes in Env, allowing coreceptor binding, which is mediated in part by the V3 loop of Env (3). To deliver the viral payload into cells, HIV Env, comprised of gp120 and gp41 subunits (1), first attaches to the host cell, binding CD4 (2). However, attachment factors differ from receptors in that they are not essential, and although they augment infection in vitro, their physiologic role in vivo remains unclear. HIV attachment to the host cell via any of these factors likely brings Env into close proximity with the viral receptor CD4 and coreceptor, increasing the efficiency of infection ( Fig. 2009) or pattern recognition receptors such as dendritic cell–specific intercellular adhesion molecular 3-grabbing non-integrin (DC-SIGN) ( Geijtenbeek et al. 2001), or can result from more specific interactions between Env and α4β7 integrin ( Arthos et al. Attachment can be relatively nonspecific, with Env interacting with negatively charged cell-surface heparan sulfate proteoglycans ( Saphire et al. First, virions must bind to the target cell, with this being mediated either by the viral envelope (Env) protein or host cell membrane proteins incorporated into the virion with any one of a number of various cell attachment factors.
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The intricate series of protein–protein interactions that ultimately results in virus infection can be divided into several phases, some of which are essential and others that may serve to modulate the efficiency of the process. HIV entry, the first phase of the viral replication cycle, begins with the adhesion of virus to the host cell and ends with the fusion of the cell and viral membranes with subsequent delivery of the viral core into the cytoplasm.
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