Secretion Of Extracellular Matrix Components Research Articles

Regulation of Membrane Fusion in Secretory Exocytosis

Exocytosis is a ubiquitous cell function that involves the intracellular transport and fusion of membrane-bound cytoplasmic vesicles or granules with the plasma membrane. Once the cytoplasmic granule and the plasma membrane have fused, their membranes are continuous and the granule interior now faces outside the cell. Hence, exocytosis serves both to incorporate membrane proteins into the plasma membrane and to secrete the contents of the granule. Since this review will focus on secretion, the membrane undergoing fusion with the plasma membrane will be referred to as the secretory organelle (or granule), and the end result of exocytosis will be referred to as secretion. There appear to be two pathways for exocytosis, constitutive and regulated (24). Constitutive exocytosis, which occurs independently of extracellular stimuli and proceeds at a steady rate, is thought to be responsible for the insertion of plasma membrane proteins and secretion of extracellular matrix components. In contrast, regulated exocytosis, which is triggered by ex­ tracellular stimuli, is responsible for episodic secretion of hormones, neurotransmitters, digestive enzymes, etc. In addition, regulated exocytosis provides a mechanism for the rapid insertion of membrane transport proteins into the plasma membrane (38). The basic scheme outlined by Palade (50) is still adequate to describe the overall morphological sequence of events in exocytosis. This sequence in­ cludes: (a) translocation of the secretory organelle from its point of formation,