Figure 7.

Analytical ultracentrifugation of the trimeric complex shows a 1:1:1 stoichiometry.A and B, results of the species analysis obtained by direct fitting the sedimentation coefficient s and diffusion coefficient D of each observed particle population to the Lamm equation (41) at each loading concentration. Only species classes 1–3 are shown. Shown are experimental sedimentation coefficient s_e (A) and D_e (B) converted to hydrodynamic radius R_h versus loading concentration of species classes 1 (black), 2 (red), and 3 (green). Vertical error bars, 95% confidence intervals of the fitted parameter (s_e or R_h). The values were then extrapolated to zero concentration using an unweighted linear fit (continuous line), yielding $s_e^0$ and $R_h^0$. The shaded area shows the 95% confidence interval of the extrapolation. C, conversion of $s_e^0$ and $D_e^0$ to mass. The conversion relies on the partial specific volume $\overline{v}$, which depends on the ratio of the components of the complex that is shown at the bottom of the plot. Multiples of the same ratio have the same $\overline{v}$; the corresponding mass ladders are shown as golden rungs. Due to the ambiguity of $\overline{v}$, multiple solutions are possible. From the absorbance optics, we know that species 1–3 must contain RNA. All solutions without RNA and solutions that do not intersect with a mass ladder rung can be excluded (gray bars). We have marked the solution for each species class we deem the most likely with a blue bar. Black bars show alternate possible solutions. The error bars represent 95% confidence intervals, which are based on the experimental uncertainties of $s_e^0$, $D_e^0$, and the solvent density and viscosity.