PROTEIN TRACKING AND DETECTION OF PROTEIN MOTION USING ATOMIC-FORCE MICROSCOPY

Height fluctuations over three different proteins, immunoglobulin G, u rease, and microtubules, have been measured using an atomic force micr oscope (AFM) operating in fluid tapping mode. This was achieved by usi ng a protein-tracking system, where the AFM tip was periodically repos itioned above a single protein molecule (or structure) as thermal drif ting occurred. Height (z-piezo signal) data were taken in 1- or 2-s ti me slices with the tip over the molecule and compared to data taken on the support. The measured fluctuations were consistently higher when the tip was positioned over the protein, as opposed to the support the protein was adsorbed on, Similar measurements over patches of an amph iphile, where the noise was identical to that on the support, suggest that the noise increase is due to some intrinsic property of proteins and is not a result of different tip-sample interactions over soft sam ples. The orientation of the adsorbed proteins in these preliminary st udies was not known; thus it was not possible to make correlations bet ween the observed motion and specific protein structure or protein fun ction beyond noting that the observed height fluctuations were greater for an antibody (anti-bovine IgG) and an enzyme (urease) than for mic rotubules.