Figure 4.

MAPs Specific of Soluble Tubulin Prevent Microtubule Assembly

(A) Composite figure of the structure of tubulin in complex with MAPs specific of soluble tubulin. The cellular proteins RB3 and CPAP and the bacterial protein CopN would overlap on tubulin, whereas TTL binds at a distinct site. Note that the SLD of RB3 binds two tubulin molecules (see Figure 1A), only one being shown here.

(B) Schematic drawing illustrating the specific effect of CopN (pink star) as an inhibitor of the microtubule plus end elongation. At that end, both free and CopN-associated tubulin molecules can elongate free protofilaments (right protofilament). Once a CopN molecule is incorporated (left protofilament), it blocks the binding of additional tubulins and impairs microtubule elongation through a capping mechanism. In contrast, at the minus end, the tubulin-CopN complex cannot be incorporated and elongation will occur if enough free tubulin is available. At that end, inhibition will be observed at high CopN concentrations through a tubulin sequestration mechanism. The steric conflict that prevents TTL from binding to the microtubule (cyan stars) is also illustrated. The microtubule is schematized by two protofilaments of three tubulins.