Abstract
Growing microtubules are protected from depolymerization by the presence of a GTP or GDP/Pi cap. End-binding proteins of the EB1 family bind to the stabilizing cap, allowing monitoring of its size in real time. The cap size has been shown to correlate with instantaneous microtubule stability. Here we have quantitatively characterized the properties of cap size fluctuations during steady-state growth and have developed a theory predicting their timescale and amplitude from the kinetics of microtubule growth and cap maturation. In contrast to growth speed fluctuations, cap size fluctuations show a characteristic timescale, which is defined by the lifetime of the cap sites. Growth fluctuations affect the amplitude of cap size fluctuations; however, cap size does not affect growth speed, indicating that microtubules are far from instability during most of their time of growth. Our theory provides the basis for a quantitative understanding of microtubule stability fluctuations during steady-state growth.
Highlights
Growing microtubules are protected from depolymerization by the presence of a GTP or GDP/Pi cap
Mean growth speeds were determined by mean displacement (MD) analysis (Fig. 1D, Middle and SI Materials and Methods) and the diffusion constant was quantified with mean-squared displacement (MSD) analysis (Fig. 1D, Bottom and SI Materials and Methods), characterizing the diffusion-with-drift process
Using correlation analysis we have characterized the properties of these fluctuations and have found that the amplitude and characteristic timescale of the cap size fluctuations can be understood quantitatively based on a simple kinetic reaction network describing cap formation
Summary
Growing microtubules are protected from depolymerization by the presence of a GTP or GDP/Pi cap. Previous studies [13, 19, 22, 23] have demonstrated how the three kinetic rate constants of the cap reaction network can be derived from the time-averaged properties of two observables: the growth trajectory of the microtubule and the spatial distribution of its cap sites.
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