**Figure 3**
(*a*) Radially averaged scattering intensities of MTs and MT–lipid complexes (solid symbols), using DOTAP/DOPC solutions with* x*_{CL} = *N*_{CL}/(*N*_{CL} + *N*_{NL}) as indicated in the figure. *R*_{CL/T} = *N*_{CL}/*N*_{T} is given by *R*_{CL/T} = 160*x*_{CL}, corresponding to the point at which the total amount of lipid is exactly enough to coat each MT with a bilayer. For *x*_{CL} = 0.1 two scans are shown, 2 h and 60 h after preparing the sample. The broken curves are examples of the assumed background (Andreu *et al.*, 1992; Fernando-Diaz *et al.*, 1996; Needleman *et al.*, 2004*b*). (*b*) Scattering data from (*a*), following background subtraction (open symbols). The solid curves are the fitted scattering models. Their χ^{2} values are as in the figure. (*c*) The variation of the radial electron density, Δρ(*r*), relative to water (broken lines), of the MTs and complex walls, obtained from fitting the scattering data in (*b*) to models of concentric cylinders. *r* is the distance from the center of the cylinders. The values for the tubulin oligomer coverage *f,* as obtained from the nonlinear fit, are indicated on the right side. The schematic represents a perpendicular cut through the MT–membrane–tubulin complex wall, corresponding to Δρ(*r*). (*d*) State diagram: *f* as a function of *x*_{CL} or the membrane charge density, σ, when lipids can fully cover the MTs. Each data point is based on scattering data and models as in (*a*)–(*c*). Solid squares correspond to *R*_{CL/T} = 160*x*_{CL}. Open symbols represent different *R*_{CL/T} values, as indicated in the figure. For 0 < *x*_{CL} ≤ 0.1 we initially get *f* = 0 (corresponding to the BOR structure). Over time (Δ*t* ~ 60 h), we obtain LPNs with higher *f* values as indicated by the arrows. |