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Order, Disorder, and Perturbations in Actin-Aldolase Rafts

Catherine Sukow ^* and David J. DeRosier 

^* The Graduate Program in Biophysics, Brandeis University, Waltham, Massachusetts 02454; and The W. M. Keck Institute for Cellular Visualization, The Department of Biology, and The Rosenstiel Basic Medical Sciences Research Center, MS029, Brandeis University, Waltham, Massachusetts 02454

Correspondence: Address reprint requests to David J. DeRosier, The W. M. Keck Institute for Cellular Visualization, The Dept. of Biology, and The Rosenstiel Basic Medical Sciences Research Center, MS029, 415 South St., Brandeis University, Waltham, MA 02454. Tel.: 781-736-2494; Fax: 781-736-2405; E-mail: derosier{at}brandeis.edu.

Actin-aldolase rafts provide insights into the use of rafts as models for three-dimensional actin bundles. Although aldolase has three twofold axes, filaments in actin-aldolase rafts were not strictly related by a twofold axis. Interfilament angles were on average +15° off the expected 180°, and most rafts appeared handed; that is, rows of cross-bridges were tilted in a clockwise direction off the perpendicular. We can account for both the deviation of the angle from 180° and the handedness of the rafts by a steric constraint due to the lipid layer. We further found that the axial spacings of cross-bridges varied significantly from raft to raft. We suggest that this difference arises from variations in the twist of the filaments that nucleate raft formation; that is, filaments added to a raft adopt the symmetry of those in the raft. We conclude that the organization of filaments in rafts can be modulated by outside factors such as the lipid layer and that the variable twist of filaments in the nucleating core of the raft are imposed on all the filaments in the raft. These results provide a measure of the potential for polymorphism in actin assemblies.