Force measurements for membrane protein manipulation

The force curve measurement mode of the atomic force microscope (AFM) enables us to measure hitherto unobservable mechanical properties of nanometer sized biological specimens. By applying this mode, we attempted to conduct such mechanical manipulations of membrane proteins as: (1) measurement of the separation force between a membrane bound receptor and a covalently cross-linked ligand molecule on the AFM tip; and (2) extraction of membrane proteins after harnessing them on a modified tip with covalent cross-linkers. Since the limiting tensile force of the covalent system used in our experiment was a crucial factor for successful manipulations, we first estimated the force to terminate the covalent cross-linking system at the single molecular level to be 1.6–1.7 nN, based on our previous data. The method was then applied to measure the force required to separate α₂-macroglobulin (α₂-M) from its receptor on the cell membrane using an AFM tip coated with the receptor binding form of the protein. From a bimodal distribution of rupture force, we obtained an average value of 120 pN as the force to separate a non-covalent association of α₂-M with its receptor. When modified tips with covalent cross-linkers aimed at amino groups on the cell surface were used, distribution of the rupture force shifted toward higher values, with a peak in the histogram ≈400–500 pN. Since the force to sever covalent cross-linking system was 1.6–1.7 nN, the observed force was ascribed to the force required to extract membrane proteins from the cell membrane after covalent bond formation.