Fig. 3. XMAP215 binds partial tubulin protofilament rings. Tubulin (4 µM) was incubated with XMAP215 (0.4 µM) in 0.1xBRB80 and 15% glycerol for 1 hour on ice before rotary shadowing. Under these conditions, a fraction of the tubulin assembled into partial rings (top row). In the absence of XMAP215, rings were not detected and the observed molecules were approximately the size of single tubulin dimers (middle row). Tubulin could be assembled into partial rings without XMAP215 (bottom row) by increasing the glycerol concentration (30%) and incubation time (several hours). Scale bar: 100 nm.