Porosomes or fusion pores are cup-shaped structures in the cell membranes of eukaryotic cells where vesicles dock in the process of vesicle fusion and secretion.[1][2] These structures are about 150 nanometers in diameter and contain many different types of protein, especially SNARE proteins that mediate the docking and fusion of the vesicles with the cell membrane. Once the vesicles have docked with the SNARE proteins, they swell, which increases their internal pressure. They then fuse with the membrane, and these pressurized contents are ejected from the cell.[3] Fusion occurs via full collapse fusion or kiss-and-run fusion.
Porosomes vary in size depending on what cell they are on; pancreatic porosomes range from 100 nm to 180 nm in diameter while in neurons they range from 10 nm to 15 nm (about 1/10 the size of pancreatic porosomes). When a secretory vesicle containing v-SNARE is located opposite to the porosome containing t-SNARE, membrane continuity (ring complex) is formed between the two. The size of the t/v-SNARE complex is directly proportional to the size of the vesicle. These vesicles contain dehydrated proteins (non-active) which are activated once they are hydrated. Once the vesicle and the porosome form a complex, GTP is needed for active transport through the water channel into the vesicle and to turn on the ion channels. This results in vesicle swelling and an increase in turgor pressure which, in turn, result in the secretion of the contents.[4]
Generally the vesicles are opened and closed by actin, but neurons require a fast response therefore they have central plugs that open to release contents and close to stop the release (the composition of the central plug is yet to be discovered).[5]
The porosome was discovered in the early to mid-1990s by a team led by Professor Bhanu Pratap Jena at Yale University School of Medicine, using atomic force microscopy.[1] Dr. Bhanu Jena is the "Distinguished Professor of Physiology" at the Wayne State University School of Medicine.
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