Figure 1. Models for fusion-mediated release of an HIV-1 content marker produced upon virus maturation.

Shown are fully (A, B) and partially (C) matured particles originating from an immature HIV-1 virion (lower left). (A) A virus with mature “leaky” capsid fully releases the content marker (iGFP, green) through a fusion pore. (B) A sealed mature capsid may release iGFP after separating from the viral envelope and uncoating. An extreme case when all iGFP molecules are trapped within the capsid is illustrated. A more likely outcome of maturation is entrapment of a fraction of iGFP in the capsid. (C) A partially matured particle contains free iGFP that is released through a fusion pore, whereas the remnants of immature capsid (Gag-iGFP) are not released. Illustrated are the virus fusion events with the plasma membrane, whereupon the viral membrane marker (red) disappears as a result of dilution.