Tubular arrays of the actin-DNase I complex induced by gadolinium

We describe the preparation and structural analysis of ordered tubular arrays of the actin-DNase I complex. These structures consist of helically stacked rings; each ring is 73 A thick, has a 240 A outer and a 120 A inner diameter, and has 7-fold rotational symmetry. The actin-DNase I complex forms tubes under conditions in which actin alone aggregates into crystalline sheets-i.e., in the presence of the trivalent cation gadolinium. Moreover, upon addition of an equimolar amount of DNase I, crystalline actin sheets are slowly converted to tubes. The rings making the tubes contain a radial dyad axis that may be identical to the dyad axis of the unit cell of the crystalline actin sheet. Evidence is presented for this identification, which in turn allows tentative assignment of actin- and DNase I-containing regions in three-dimensional reconstructions of the rings. The structural analysis presented here may be useful in aligning available three-dimensional molecular models of actin determined from crystals of the actin-DNase I complex and from crystalline actin sheets with each other and ultimately within the biologically important actin filament.