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The atomic force microscope (AFM) is a remarkably simple instrument that can measure forces down to piconewtons and can resolve force changes caused by the displacement of its probe by a fraction of a nanometer. Furthermore, these measurementscanbedoneunderwater,allowingforthestudy of biological material under conditions that resemble thosein vivo.TheAFMfunctionslikeaminiaturephonograph.Asharp tip,mountedonacantilever,interactswiththesample,causing minutedeflectionsthatcanbecalibratedasaforce.TheAFM, in its force-measuring mode, has been applied to the study of molecular mechanics. For example, the laboratory of Her- mann Gaub in Munich succeeded in the remarkable feat of measuring the adhesion force between single avidin-biotin pairs ('160 pN; ref. 1). These measurements were indepen- dently confirmed by molecular dynamic simulations of the avidin-biotininteractionthatrevealedtheatomictrajectoryof a rupture and predicted the associated forces (2). Hence, the techniquesofmolecularmechanicsnowrivalthecapabilitiesof patch-clamp techniques (3) in describing the activity of single proteins. The patch-clamp technique seals a lipid membrane, containingionchannelproteins,ontoaglasspipetteelectrode. Ioniccurrentsflowingthroughasingleionchannelproteinare then easily measured, reporting on the conformational states of the transmembrane protein. In contrast to the patch-clamp technique, useful mainly for the characterization of ion chan- nels, molecular mechanics studies can be done on single proteins or more complex structures that do not carry an associated electrical activity. ThemostfamiliaruseoftheAFMisincreatingtopographic imagesofasample.ThisisdonebyscanningtheAFMtipover the sample and recording the z axis displacement required to maintain a constant contact force. The nanometer resolution of the AFM imaging technique has been used to examine the structural features of proteins, producing data that is compa- rable to that obtained by x-ray crystallography (4). However, the biggest promise of the AFM technique is in the measure- ment of the activity of single protein structuresin vivoduring signaling events or during other types of cellular activities. Indeed, we may be getting close. A recent example demon- stratedthatitispossibletoobserveconformationalchangesin the structure of a single nuclear pore in Xenopus oocyte nuclear envelopes (5). However, these measurements were doneinfixedtissue.RecentworkfromthelaboratoryofBhanu

Cellular and molecular mechanics by atomic force microscopy: Capturing the exocytotic fusion pore in vivo ?