The nanoscale molecular morphology of docked exocytic dense-core vesicles in neuroendocrine cells

The molecular organization of exocytic vesicles regulates their transport and fusion. Prasai, Taraska, and colleagues use correlative light and electron microscopy, along with 3D tomography and gold labeling, to directly map proteins on single exocytic organelles at the plasma membrane. Rab-GTPases and their interacting partners are key regulators of secretory vesicle trafficking, docking, and fusion to the plasma membrane in neurons and neuroendocrine cells. Where and how these proteins are positioned and organized with respect to the vesicle and plasma membrane are unknown. Here, we use correlative super-resolution light and platinum replica electron microscopy to map Rab-GTPases (Rab27a and Rab3a) and their effectors (Granuphilin-a, Rabphilin3a, and Rim2) at the nanoscale in 2D. Next, we apply a targetable genetically-encoded electron microscopy labeling method that uses histidine based affinity-tags and metal-binding gold-nanoparticles to determine the 3D axial location of these exocytic proteins and two SNARE proteins (Syntaxin1A and SNAP25) using electron tomography. Rab proteins are distributed across the entire surface and t-SNARE proteins at the base of docked vesicles. We propose that the circumferential distribution of Rabs and Rab-effectors could aid in the efficient transport, capture, docking, and rapid fusion of calcium-triggered exocytic vesicles in excitable cells.

Automated summary

By utilizing correlative super-resolution light and electron microscopy, researchers have mapped the location and organization of Rab-GTPases and their effectors on exocytic vesicles, shedding light on their functional roles within cells.