Electron tomography of the cortical actin network surrounding secretory granules docked at the plasma membrane. (A) Electron micrograph showing a plasma membrane prepared from a nicotine-stimulated cell. Secretory granules docked at the plasma membrane were surrounded by a filamentous network. (B) A 3D model of the granule in A showing the close interaction between docked secretory granules and the cortical F-actin network covering the inner face of the plasma membrane. False colors were applied using a color code related to the topographic height (shown on the right), with the plasma membrane sheets as the lowest plane (Video 1). (C–H) Series of tomographic slices of the docked granule in A. The first (Top) and last sections (Bottom) correspond, respectively, to the farthest and closest parts of the docking region of secretory granules at the plasma membrane. Actin colloidal gold immunostaining (arrows) served as a fiducial marker for image alignment during tomographic reconstruction. The inset in E is a 2.5× enlargement of the granule whose limit is shown as a broken black line. The asterisks indicate the protrusions that correspond to anchored F-actin. SG, secretory granule. (I) Surface-rendered view of a subtomogram corresponding to an equatorial section of the granule shown in A. The 3D representation is associated with the three tomographic sections (left) showing different branched anchoring structures that are connected to the granule. (J) Surface-rendered view of a subtomogram corresponding to a longitudinal section of the granule shown in A. The volume eraser tool in UCSF Chimera was used to perform curettage inside the granule until reaching the membrane. The topography of the granule surface is displayed in purple. The excess removed by curettage is shown in green. The path followed by the actin filaments is represented by the broken yellow lines. Bars: (A) 200 nm; (C) 100 nm; (E, inset) 25 nm; (I and J) 25 nm.