Imaging Globular and Filamentous Proteins in Physiological Buffer Solutions with Tapping Mode Atomic Force Microscopy

Abstract

Two different types of proteins were imaged in buffer solution with tapping mode atomic force microscopy (AFM) in liquids : the globular proteins lysozyme and monomeric actin (G-actin) ; the filamentous proteins actin (F-actin) and microtubules. To calibrate the AFM in contact and tapping modes in liquids, a sample was prepared with features that are comparable to the height of a single protein molecule: steps in mica with a height of 1 nm. Single globular molecules of lysozyme and G-actin could be readily imaged in physiological buffer at pH 7 and lower, whereas F-actin could only be imaged stably without any visible damage at around pH 6. The helical pitch of the actin filaments was measured to be 37 nm, which is in good agreement with data from X-ray diffraction and transmission electron microscopy (TEM). The negatively charged microtubules could not be imaged on freshly cleaved mica ; instead, a method was established to adsorb them to silanized glass. Both protein types could be imaged stably with loading forces of about 200 pN. The height of the proteins was larger than the expected height measured by X-ray diffraction on protein crystals. Mechanical properties and/or electrostatic interactions may contribute to the image formation. Further work is needed to understand the height measured by tapping mode in liquids. We show here that single, globular protein molecules and protein filaments can be imaged easily and stably in buffer solution.