FIGURE 1:

Classical understanding of MT polymer assembly behavior. See Table 1 for additional description of the CC measurements depicted here. [Free tubulin] is the concentration of tubulin dimers in solution, [polymerized tubulin] is the concentration of tubulin dimers in polymerized form, and [total tubulin] = [free tubulin] + [polymerized tubulin]. (A) In a competing (closed) system, [total tubulin] is held constant over time and MTs compete for tubulin. As typically presented in textbooks, the CC can be measured in a competing system by observing either the concentration of total tubulin at which MT polymer appears (Q1) or the concentration of free tubulin left in solution once the amount of polymer has reached steady state (Q2). (B) In a noncompeting (open) system, [free tubulin] is held constant over time. In such a system, CC is considered to be the minimum concentration of tubulin necessary for MT polymers to grow, which is estimated by measuring the growth rate of individual filaments (V_g) and extrapolating back to V_g = 0 (Q3). (C) In dilution experiments, MTs are grown under competing conditions until the system reaches polymer-mass steady state and then diluted into various [free tubulin]. The initial rate of change in [polymerized tubulin] is measured. Here, CC is the concentration of dilution [free tubulin] at which the rate of change in [polymerized tubulin] is zero (i.e., the dilution [free tubulin] at which the net flux of tubulin into and out of MT polymer is zero) (Q4). (D) Summary table of the definitions of the experimentally measurable quantities Q1–Q4 depicted in panels A–C. (E) Individual MTs exhibit a behavior called dynamic instability (DI), in which the individuals undergo phases of growth and shortening separated by approximately random transitions termed catastrophe and rescue. (F) Table of definitions of DI parameters (four measurements commonly used to quantify DI behavior).