Structure and Stability of Self-Assembled Actin-Lysozyme Complexes in Salty Water

Interactions between actin, an anionic polyelectrolyte, and lysozyme, a cationic globular protein, have been examined using a combination of synchrotron small-angle x-ray scattering and molecular dynamics simulations. Lysozyme initially bridges pairs of actin filaments, which relax into hexagonally coordinated columnar complexes comprised of actin held together by incommensurate one-dimensional close-packed arrays of lysozyme macroions. These complexes are found to be stable even in the presence of significant concentrations of monovalent salt, which is quantitatively explained from a redistribution of salt between the condensed and the aqueous phases.

Figure 1

(a) Synchrotron 2D x-ray diffraction pattern of partially aligned actin-lysozyme bundles, formed in a solution containing 150 mM KCl. (b) 1D integrated slices along the $q_z$ and $q_r$ directions with arrows marking the actin-actin close-packed bundling peak (1), the actin helix form factor (2), and the lysozyme-lysozyme correlation peak (3). (c) Proposed structure of actin-lysozyme composite bundles (side and end views): lysozyme (orange) is close packed in threefold symmetric sites between actin filaments (blue).

Figure 2

Osmotic pressure of a hexagonally coordinated bundle of actin filaments without added salt, as determined from MD simulations. Inset: lysozyme pair correlation function along the filament axis. For discussion see the text.

Figure 3

Contour plots showing the lysozyme distribution in actin-lysozyme complexes without added salt. Darker shading corresponds to higher concentrations. In the equilibrium configuration (a), lysozyme is predominantly located in the threefold interstitial regions. The free-energy inflection point occurs at a slightly expanded lattice (c), in which lysozyme is depleted from these regions and occupies the bridging regions instead. Panels (b) and (d) are the counterparts of panels (a) and (c), respectively, in the absence of electrostatic forces.

Figure 4

Series of diffraction data showing the evolution of bundle structure as a function of (a) NaCl and (b) KCl concentration with maximum bundling occurring around 120 mM. (c) Simulated (○) bundle vs bulk ion concentrations for a stabilized actin-lysozyme complex. Salt partitions into different concentrations inside and outside the bundle. The dashed line is a guide to the eye.