Focal adhesion membrane is dotted with protein islands and partitioned for molecular hop diffusion

Using the ultrafast camera system and new theories for hop diffusion described in the companion paper, we for the first time demonstrated that membrane molecules undergo hop diffusion among the compartments in the bulk basal plasma membrane (PM), with virtually the same compartment sizes (108 nm) as those in the bulk apical PM and the same dwell lifetimes within a compartment (10 and 24 ms for the phospholipid and transferrin receptor [TfR], respectively), suggesting that the basic structures and molecular dynamics are very similar in the bulk regions of the apical and basal PMs. Ultrafast PALM and single-molecule imaging revealed that the focal adhesion (FA) is mostly a fluid membrane, partitioned into ~74-nm compartments where TfR and β3 integrin undergo hop diffusion, and that the FA membrane is sparsely dotted with 51-nm diameter paxillin islands, where many other FA proteins probably assemble (compartmentalized archipelago model). β3 integrin intermittently associates with the paxillin islands, dynamically linking them to the extracellular matrix.