Energy landscape of aptamer/protein complexes studied by single-molecule force spectroscopy

Aptamers are single-stranded nucleic acid molecules selected in vitro to bind to a variety of target molecules. Aptamers bound to proteins are emerging as a new class of molecules that rival commonly used antibodies in both therapeutic and diagnostic applications. With the increasing application of aptamers as molecular probes for protein recognition, it is important to understand the molecular mechanism of aptamer-protein interaction. Recently, we developed a method of using atomic force microscopy (AFM) to study the single-molecule rupture force of aptamer/protein complexes. In this work, we investigate further the unbinding dynamics of aptamer/protein complexes and their dissociation-energy landscape by AFM. The dependence of single-molecule force on the AFM loading rate was plotted for three aptamer/protein complexes and their dissociation rate constants, and other parameters characterizing their dissociation pathways were obtained. Furthermore, the single-molecule force spectra of three aptamer/protein complexes were compared to those of the corresponding antibody/protein complexes in the same loading-rate range. The results revealed two activation barriers and one intermediate state in the unbinding process of aptamer/protein complexes, which is different from the energy landscape of antibody/protein complexes. The results provide new information for the study of aptamer-protein interaction at the molecular level.