Regulation of Membrane Fusion during Exocytosis

Abstract

Publisher Summary This chapter describes the regulation of membrane fusion during exocytosis. Exocytosis involves the fusion of a secretory organelle with the cell membrane. It is important to discriminate between regulated (triggered) exocytosis and constitutive (untriggered) exocytosis. Specific signals and signal-transfer mechanisms occur only with regulated exocytosis. The mechanisms of membrane-fusion regulation are different from these two types of exocytosis. Exocytosis is of the PF–PF fusion type; plasmatic membrane faces fuse. It always follows the focal fusion scheme: fusion is restricted to a small area of ≤ 10 nm, before the exocytotic opening expands to release the secretory contents. Adenosine triphosphate (ATP) is required for intracellular transport of secretory organelles; it might also be required for a rearrangement of microfilamentous elements on the cell periphery during secretion. The protein phosphorylation, another phenomenon generally occurring during secretory activity, clearly requires ATP. The hydrolysis of membrane-bound ATP has been proposed to be a fusogenic process. Guanosine triphosphate (GTP) analogs stimulate membrane fusion in some systems; they might act by various mechanisms.