Figure 1.

Trajectories of single U1 snRNPs in a living cell nucleus. (A) Trajectories (black dots connected by black lines) of numerous single U1snRNPs extracted from a single-molecule movie, which was recorded at 200 Hz (Supplementary Movie 1, Online Supplementary Data). The trajectories were plotted over an image of the ASF/SF2-GFP, which was transiently expressed in the respective HeLa cell nucleus. Here, all nuclear trajectories were shown; however, those close to the borders were not evaluated (see Materials and Methods). The brighter spots marked by dark gray lines indicated the positions of speckles. The approximate position of the nuclear envelope was deduced from the limits of ASF/SF2-GFP fluorescence, and indicated by the white line. Mobile U1 snRNPs as well as transiently immobilized RNPs could well be distinguished within nucleoplasm and speckles. The white bar corresponded to 2 μm. The white box marks a region containing a single trajectory, which was scrutinized in B to D. (B) The short image sequence of the trajectory marked in A. The size of a single image was 8 μm². (C) Magnified view of the positions, at which the U1 snRNP was observed, revealing that the RNP moved to a specific site, resided there for a certain time and then left the site again. Please note that the complete field shown corresponds to 1 μm² only. (D) Time course of the fluorescence signal at the attachment region x = 6.77 ± 0.05 and y = 2.67 ± 0.05 μm, which indicated that the particle remained for 45 ms at the specific nucleoplasmic position before leaving.