Structures of large T antigen at the origin of SV40 DNA replication by atomic force microscopy

For inorganic crystals such as calcite (CaCO3), Atomic Force Microscopy (AFM) has provided surface structure at atomic resolution (Ohnesorge and Binnig, 1993). As part of a broad effort to obtain high resolution for an individual protein or protein assembly (Binnig et al., 1986; Rugar and Hansma, 1990; Radmacher et al., 1992), we applied AFM to study the ATP-dependent double hexamer of SV40 large T antigen, which assembles around the viral origin of DNA replication. Multimeric mass has been determined in two-dimensional projected images by Scanning Transmission Electron Microscopy (STEM) (Mastrangelo et al., 1989). By AFM, if the DNA-protein preparation has been stained positively by uranyl acetate, the contour at the junction between hexamers is visible as a cleft, 2-4 nm deep. The cleft, whether determined as a fraction of height by AFM or as a fraction of mass thickness by STEM, is of comparable magnitude. On either side of the cleft, hexamers attain a maximum height of 13-16 nm. Monomers found in the absence of ATP show heights of 5-7 nm. Taken together, the z coordinates provide a surface profile of complete and partial replication assemblies consistent with the spatial distribution of recognition pentanucleotides on the DNA, and they contribute direct geometrical evidence for a ring-like hexamer structure.