Nanoscale 3D contour map of protein assembly within the astrocyte porosome complex

The astrocyte porosome complex, the secretory machinery at the plasma membrane of astrocytes, is a 10-15 nm cup-shaped lipoprotein structure possessing a central plug. Since the porosome is a membrane-associated, multi-protein complex, it has precluded the generation of 3D crystals for X-ray diffraction studies, nor structural analysis at the atomic level using the solution NMR. These limitations were partially overcome in the current studies, furthering our understanding of the porosome structure in astrocytes. Using atomic force microscopy, electron microscopy, and electron density and 3D contour mapping, finally provides at the nanoscale, the structure and assembly of proteins within the astrocyte porosome complex. Results from this study demonstrate a set of protein units lining the porosome cup, each connected via spoke-like elements to a central plug region within the structure. In contrast to the neuronal porosome, which possess eight globular proteins at the outer rim of the complex, the porosome complex appear to possess 12 such globular structures. Nature has designed the porosome as the universal secretory machinery, but has fine-tuned its use to suite secretion from different cell types. The isolation of intact astrocyte porosomes for near-atomic resolution using cryo-electron diffraction measurements, is finally possible.