secretion vesicles

There are two main classes of traffic intermediates that operate in intracellular trafficking pathways: small round vesicles (SRVs), and large pleiomorphic carriers (LPCs).

While both are essential, the large pleiomorphic carriers (LPCs) appear to be responsible for moving the bulk of the secretory traffic between distant compartments.

LPCs are much larger and more variable in shape than vesicles, and they have evident interconnected tubular and saccular/cisternal components.

LPCs appear to form by en bloc extrusion and cleavage of large membrane areas of the donor organelle.

Secretion vesicles docking is thought to be regulated in part by the specific interactions of a vesicle-associated membrane protein (VAMP), or synaptobrevin (MIM.185880), with the presynaptic plasma membrane proteins syntaxins and SNAP25 (MIM.600322).

Function

Regulated secretion of hormones occurs when a cell receives an external stimulus, triggering the secretory granules to undergo fusion with the plasma membrane and release their content into the extracellular milieu.

The formation of a mature secretory granule (MSG) involves a series of discrete and unique events such as protein sorting, formation of immature secretory granules (ISGs), prohormone processing and vesicle fusion.

Regulated secretory proteins (RSPs), the proteins stored and secreted from MSGs, contain signals or domains to direct them into the regulated secretory pathway.

Specific domains in RSPs are involved in sorting and aggregation suggest that the cell-type-specific composition of RSPs in the trans-Golgi network (TGN) has an important role in determining how the RSPs get into ISGs. The realization that lipid rafts are implicated in sorting RSPs in the TGN and the identification of SNARE molecules represent further major advances in our understanding of how MSGs are formed.

Cells

 endocrine cells
 neurons

Features

 molecular biology of vesicles fusion with plasma membrane
 molecular biology of endocrine secretion
 regulated secretory proteins
 immature secretory granules
 mature secretory granule

Vesicle associated proteins

 vesicular transporter proteins

• VGLUTs
• VGAT
• VAChT

 SNAREs and SNARE-associated proteins

• VAMPs
• syntaxins
• SNAPs
• alpha/beta-SNAP
• NSF
• complexin
• Munc-18
• VPS-45

 other vesicle associated proteins

• synaptophysins
• synaptogyrin
• synapsin
• synaptotagmins
• Rabs
• dynamin
• clathrin

References

 Luini A, Ragnini-Wilson A, Polishchuck RS, De Matteis MA. Large pleiomorphic traffic intermediates in the secretory pathway. Curr Opin Cell Biol. 2005 Aug;17(4):353-61. PMID: 15993575

 Robinson MS. Adaptable adaptors for coated vesicles. Trends Cell Biol. 2004 Apr;14(4):167-74. PMID: 15066634

 Guo W, Novick P. The exocyst meets the translocon: a regulatory circuit for secretion and protein synthesis? Trends Cell Biol. 2004 Feb;14(2):61-3. PMID: 15106610

 Solimena M, Gerdes HH. Secretory granules: and the last shall be first... Trends Cell Biol. 2003 Aug;13(8):399-402. PMID: 12888291

 Tooze SA, Martens GJ, Huttner WB. Secretory granule biogenesis: rafting to the SNARE. Trends Cell Biol. 2001 Mar;11(3):116-22. PMID: 11306272