Probing Biomaterials with the Atomic Force Microscope

Recent AFM research in our laboratory has covered such diverse biomaterials as laminin and other macromolecules from basement membranes (Fig. 1) (1), DNA condensed for gene therapy (Fig. 2) (2), DNA-protein complexes in the yeast kinetochore (Fig. 3) (3), and biofilms of the bacterium Pseudomonas putida (Fig. 4) (4).Laminin is a major protein of basement membranes. When analyzed by AFM in air, it shows a variety of conformations of its cruciform structure (Fig. 1A). Time-lapse images of a single laminin molecule in aqueous solution show the flexibility of the laminin arms as they move and bend (Fig. 1B)(1).AFM analysis of DNA condensed for receptor-mediated gene therapy poses a puzzle (Fig. 2)(2). Polylysine condenses DNA poorly (Fig. 2A), while AsOR-polylysine condenses DNA into compact toroids and short rods (Fig. 2B). AsOR (asialoorosomucoid) is a 38-kDa glycoprotein with a net negative charge of 5 carboxylic acid groups per molecule. The puzzle is why this negative glycoprotein, covalently attached to polylysine, can enhance the condensation of DNA so much over the condensation caused by the polycation, polylysine, alone.