The micro-mechanics of single molecules studied with atomic force microscopy

The atomic force microscope (AFM) in its force-measuring mode is capable of effecting displacements on an angstrom scale (10 Ansgtrom = 1 nm) and meas uringforces of a few piconewtons. Recent experiments have applied AFM techn iques to study the mechanical properties of single biological polymers. The se properties contribute to the function of many proteins exposed to mechan ical strain, including components of the extracellular matrix (ECM). The fo rce-bearing proteins of the ECM typically contain multiple tandem repeats o f independently folded domains, a common feature of proteins with structura l and mechanical roles. Polysaccharide moieties of adhesion glycoproteins s uch as the selectins are also subject to strain. Force-induced extension of both types of molecules with the ABM results in conformational changes tha t could contribute to their mechanical function. The force-extension curve for amylose exhibits a transition in elasticity caused by the conversion of its glucopyranose rings from the chair to the boat conformation. Extension of multi-domain proteins causes sequential unraveling of domains, resultin g in a force-extension curve displaying a sam tooth pattern of peaks. The e ngineering of multimeric proteins consisting of repeats of identical domain s has allowed detailed analysis of the mechanical properties of single prot ein domains. Repetitive extension and relaxation has enabled direct measure ment of rates of domain unfolding and refolding. The combination of site-di rected mutagenesis with AFM can be used to elucidate the amino acid sequenc es that determine mechanical stability. The AFM thus offers a novel way to explore the mechanical functions of proteins and will be a useful tool for studying the micro-mechanics of exocytosis.