A polymorphism peculiar to bipolar actin bundles.

Both muscle and nonmuscle actins produced magnesium paracrystals which we found indistinguishable from one another. Contrary to some previous reports, calcium ions caused no change in filament organization for either type of actin. The most ordered paracrystals consisted of hexagonally packed filaments with opposite polarities. We suggest that this mode of packing permits a form of disorder not previously described, which may account for some puzzling aspects of earlier observations and may prove useful in analyzing actin bundles formed, for example, with erythrocyte band 4.9 protein.