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Using this construct expressed in trans with viral Env lacking the CT domain, we show that increasing particle stiffness reduces viral entry activity in immature virions. Results In this study, we show that transmembrane-anchored Env cytoplasmic tail (CT) domain is sufficient to regulate the particle stiffness of immature HIV- 1. Using Atomic Force Microscopy (AFM), we previously discovered that HIV undergoes a “stiffness switch”, a dramatic reduction in particle stiffness during maturation that is mediated by the viral Envelope (Env) protein. Little is known about how the physical properties of viral particles change during maturation and how these changes affect the viral lifecycle. Virion stiffness regulates immature HIV- 1 entryīackground Human immunodeficiency virus type 1 ( HIV- 1) undergoes a protease-mediated maturation process that is required for its infectivity. Moreover, as a result of the low number of Env trimers on individual virions, the number of additional Env trimers per virion that is required for the optimal infectivity will depend on the inclusion of facilitating agents during infection. These results provide the first quantitative evidence that Env trimers cooperate on the virion surface to mediate productive infection by HIV- 1. In contrast, in the absence of DEAE-dextran, virion infectivity increases monotonically with gp120 density and no saturation is observed under the experimental conditions. This quantitative dependence can be described by a Hill equation, with a Hill coefficient of 2.4 ± 0.6. In the presence of DEAE-dextran, the polycation known to enhance HIV- 1 infectivity in cell culture, virion infectivity follows gp120 density as a sigmoidal dependence and reaches an apparent plateau. The resulting gp120 density per virion is then correlated with the infectivity of the virions measured in cell culture.

This method involves 1) production of a set of single-cycle HIV- 1 virions with varied density of Env on their surface, 2) site-specific labeling of Env-specific antibody Fab with a fluorophore at high efficiency, and 3) optical trapping virometry to measure the number of gp120 molecules on individual HIV- 1 virions. Here we have developed a method to examine this dependence.

Although in general it has been perceived that more Env gives rise to higher infectivity, the precise quantitative dependence of HIV- 1 virion infectivity on Env density has remained unknown. The envelope glycoprotein (Env) gp120/gp41 is required for HIV- 1 infection of host cells. Quantitative Correlation between Infectivity and Gp120 Density on HIV- 1 Virions Revealed by Optical Trapping Virometry*ĭeSantis, Michael C.