Overview of Secretory Vesicles

Constitutive secretory vesicles continuously transport molecules like extracellular matrix proteins and growth factors to the plasma membrane without specific stimuli. Regulated secretory vesicles, by contrast, transport proteins such as insulin and neurotransmitters only upon stimulation. Regulated vesicles concentrate proteins during maturation, enabling rapid, large-scale release when triggered.

Immature secretory vesicles bud from the trans-Golgi network and fuse with each other during maturation. As they fuse, V-type ATPases on the vesicular membrane increase in concentration and pump protons inward, lowering intraluminal pH. This acidic environment enables protein aggregation, forming mature secretory vesicles with concentrated protein cargo ready for exocytosis.

Chromogranins A and B are matrix proteins that bind soluble molecules within secretory vesicles, aggregating them into dense core structures. These proteins function optimally at low pH and high calcium concentration, both present in vesicles budding from the trans-Golgi network. Calcium-induced conformational changes in chromogranins enable their binding to the vesicular membrane, facilitating proper protein sorting.

The low pH maintained by V-type ATPases is essential for chromogranin function and protein aggregation within secretory vesicles. This acidic environment enables chromogranins A and B to undergo conformational changes necessary for binding and concentrating soluble molecules. The resulting protein aggregates ensure efficient delivery of large amounts of cargo to the cell surface during fusion of secretory vesicles with the plasma membrane.

Temperature-sensitive sec mutants of Saccharomyces cerevisiae, generated by Schekman and Novik in 1979, accumulated vesicles and internal membranes when non-functional at 37°C. Molecular analysis identified 23 sec genes encoding regulatory proteins involved in different secretory transport stages. Some mutants blocked vesicle fusion with the plasma membrane, while others prevented protein transport to organelles, revealing the complexity of secretion regulation.

Secretory vesicles budding from the trans-Golgi network are rich in calcium, which is abundant in the endoplasmic reticulum and Golgi complex. Calcium induces pH-dependent conformational changes in chromogranins A and B, enabling them to bind and aggregate with other vesicular matrix proteins and the vesicular membrane. This calcium-dependent aggregation is critical for sorting specific proteins into regulated secretory vesicles.

Dense core vesicles (DCVs) are membrane-bound secretory vesicles that transport hormones and neurotransmitters requiring rapid exocytosis upon stimulation. Unlike constitutively secreted proteins, DCV cargo undergoes aggregation mediated by chromogranins at low pH, creating concentrated protein cores. This dense aggregation distinguishes DCVs and enables them to release large protein quantities quickly in response to specific cellular signals.