Figure 5

Fluorescence fluctuation spectroscopy analysis of HTLV-1 Gag-based VLPs. (A). The fluorescence intensity trace shows discrete peaks, which correspond to individual VLPs diffusing through the observation volume. (B). Experimental photon counting histogram (diamonds) of the VLP sample. A fit (solid line) of the histogram to a 2-species model with background identifies the concentration and Gag copy number of the VLPs. The presence of two species indicates the existence of heterogeneity in the Gag copy number of VLPs. A weighted average of the two species leads to an average Gag copy number of 530 per VLP. The first VLP species has a copy number of 270 and a concentration of 20.5 pM. The second VLP species, which is brighter than the first, has a copy number of 800 and a concentration of 6.5 pM. (C). A fit (solid line) of the autocorrelation function (diamonds) to a diffusion model identifies an average hydrodynamic diameter of 74 nm for the VLPs.