Fig. 1. GFP-tau binds to individual MTs and induces MT bundling and the formation of neurite-like processes in Xenopus embryo cultures. (A-D) Fluorescence micrographs of fibroblast-like cells expressing GFP-tau. GFP-labeled polymeric structures can be observed in the cytoplasm. Individual polymers at the edge of the cell (marked by arrows in A) are similar in intensity and therefore are likely to represent single MTs. MT bundles can be found both in the soma (B) and inside neurite-like cell processes (C). Panels A and D show representative examples of cells demonstrating non-centrosomal and centrosomal patterns of MT organization. Bar, 10 µm. (E) Series of fluorescence images of the lamella region of a fibroblast transfected with GFP-tau. Both growing (arrows) and shortening (arrowheads) MTs are visible. Time (in seconds) is shown at the upper-right corner. Bar, 10 µm. (F) Quantitative analysis of GFP-tau binding to individual MTs. For each MT, the intensity of fluorescence was measured at distances of 1 µm (open bar) and 5-10 µm (hatched bar) from the tip. The background fluorescence at adjacent MT-free regions was subtracted from the measurements. Data from 50 MTs in seven different neurons are presented as means±s.e.m. (G) Series of fluorescence images of GFP-tau-expressing fibroblasts (elapsed time is stated in seconds). Both individual MTs and MT bundles are detected at the edge of the cell. MT bundles arise through lateral interaction between individual MTs (marked by arrows). Bar, 10 µm.