Single hepatitis-B virus core capsid binding to individual nuclear pore complexes in Hela cells

Abstract

We investigate the interaction of hepatitis B virus capsids lacking a nuclear localization signal with nuclear pore complexes (NPCs) in permeabilized HeLa cells. Confocal and wide-field optical images of the nuclear envelope show well-spaced individual NPCs. Specific interactions of capsids with single NPCs are characterized by extended residence times of capsids in the focal volume which are characterized by fluorescence correlation spectroscopy. In addition, single-capsid-tracking experiments using fast wide-field fluorescence microscopy at 50 frames/s allow us to directly observe specific binding via a dual-color colocalization of capsids and NPCs. We find that binding occurs with high probability on the nuclear-pore ring moiety, at 44 +/- 9 nm radial distance from the central axis.

FIGURE 1

Combination of a fluorescence scanning confocal optical microscope and a wide-field fluorescence microscope. The detection beam path can be switched by a flipping mirror. S, sample; O, objective; DM1, dichroic mirror; FL, flippable lens; BC, beam combiner (488/532pc, Chroma Technology); LF1, LF2, line filters; L, lens; P, periscope; NF1, NF2, notch filters; FM, flappable mirror; DM2, dichroic mirror; BP1, BP2, band-pass filters; SPCM, single-photon counting module.

FIGURE 2

Imaging of single NPCs at the ventral plane of NE by (a) scanning confocal optical microscope and (b) wide-field fluorescence microscope. Compared to scanning confocal optical microscopy, wide-field microscopy obviously suffers from a higher background while providing similar resolution. (scale bars, 1 mm).

FIGURE 3

Accumulation of HBV core capsids at the NE. Wide-field images of (a) mAb414 antibody-labeled NPCs and (b) GFP-HBV core capsids accumulated at the NE. (scale bars, 5 mm).

FIGURE 4

FCS experiment. Scheme of the confocal volume positioned at the location of an NPC showing different possible causes for fluorescence bursts.

FIGURE 5

FCS experiment. Histograms of lengths of fluorescence bursts where the confocal volume is placed (a) at the NE without an NPC, (b) at the NE in the presence of NPC, and (c) in the cytoplasm away from the nucleus. The plots in panels a and c are fitted by an exponential decay. The plot in b is fitted by a double-exponential decay, with one decay rate retained fixed with the value obtained in panel a. The insets display the respective autocorrelation curves fitted by an anomalous diffusion model. Portions of time traces showing representative events are also shown as insets.

FIGURE 6

Simulation of FCS experiment. Histograms of lengths of fluorescence bursts. Identical to the plots in Fig. 5, the plots in panels a and c are fitted by an exponential decay, whereas the plot in b is fitted by a double-exponential decay, with one decay rate retained fixed with the value obtained in panel a. The decay rates obtained are (a) , (b) and , and (c) . Sketches of the simulated structure for each situation are shown as insets.

FIGURE 7

Wide-field microscopy. HBV core capsid interaction with NE was traced in HeLa cells with antibody-labeled NPCs by obtaining sequential images at 50 frames/s. Histograms of lengths of HBV core capsid-NPC interactions during which (a) capsids bound to NE without colocalization with clearly resolved NPC and (b) capsids colocalized with an NPC. Capsids bound within a radius of 110 nm from an NPC signal are considered to have colocalized. (c) Several trajectories of capsids around NPCs overlapped to a sum of original NPC signals corresponding to the trajectories.

FIGURE 8

Movement of HBV core capsids around NPC observed by dual-color wide-field microscopy. (a) Diffusion coefficients versus duration of HBV core capsid-NPC interactions. (b) Probability density distribution for the position of the capsid binding events with respect to the central axis of NPCs in polar coordinates r and φ determined from 23 binding events that lasted at least 2 frames. Distance between contours, 1′10⁻⁴. A sketch of the NPC (see Fahrenkrog et al. (29)) is aligned to the probability density plot. The highest probability for capsid binding is found for distances of 44 ± 9 nm from the central axis of an NPC. Bindings at larger distances occur with much lower probability. 1, cytoplasmic filaments; 2, luminal domain; 3, ring moiety; 4, nuclear basket.