Optical Trapping and Multi-parameter Analysis of Single HIV-1 in Culture Media Reveal the Positive Cooperativity of Envelope Spikes in Mediating Viral Infection
Using ultrahigh resolution optical tweezers, we demonstrate that it is feasible to optically trap and manipulate single HIV-1 virion in culture media ‘contact-free’ in three dimensions under native conditions. The back-focal-plane interferometry with angstrom-level resolution measured single virion diameter in culture media with less than 7% error compared to cryoelectron microscopy. Single HIV-1 virions orient themselves in the photon electric field, which permits measurement of the overall shape of individual particles. We show that majority of virions displayed a slightly elongated shape. This eccentricity was manifested only upon virion maturation, suggesting a global shape change of HIV-1 particle upon protease cleavage of Gag polyproteins. Using a simultaneous two-photon fluorescence detection scheme with single-molecule sensitivity, we show that the number of envelope spikes that are essential for viral infection varies widely on the surface of individual virions. The efficiency of virus infection varied with the envelope content in a distinct sigmoidal dependence, which revealed a Hill coefficient of 2.9±0.8. These results suggest that multiple envelope spikes cooperate on virion surface to mediate HIV-1 infection. As a result, individual virions differ in their ability to infect host cells due to heterogeneity of envelope spike content. We hypothesize that HIV-1 virions with high envelope spike content are preferentially transmitted in human populations, consistent with recent finding (Parrish et al., PNAS 2013) that transmitted founder viruses contained 1.9-fold more envelope per particle than chronic control viruses (Supported by NIH Director’s New Innovator Award 1DP2OD008693).