Membrane fusion of secretory vesicles and liposomes Two different types of fusion

Roland Ekerdt, Gerhard Dahl, Manfred Gratzl
1981 Biochimica et Biophysica Acta - Biomembranes  
Secretory vesieles isolated from adrenal medulla were found to fuse in vitro in response to incubation with Ca 2 +. Intervesicular fusion was detected by electron microscopy and was indicated by the appearance of twinned vesieies in freeze-fractured suspensions of vesieles and in thin-sectioned pellet. Two types of fusion could be distinguished: Type I, occurring between 10-7 M and 10-4 M Ca 2 +, was specific for ea 2 +, was inhibited by other divalent cations and was abolished by pretreatment
more » ... f vesieles with glutaraldehyde, neuraminidase or trypsin. Fusion type I was linear with temperature. A second type of intervesicular fusion was elicited by Ca 2 + in concentrations higher than 2.5 mM and was morphologically characterized by multiple fusions of secretory vesicles. This type of fusion was found to be similar to fusion of liposomes prepared from the membrane lipids of adrenal medullary secretory vesieles: ea 2 + could be replaced by other divalent cations, the effect of different divalent cations was additive and pretreatments attacking membrane proteins were ineffective. Fusion type 11 of intact secretory vesieies as weil as liposome fusion was discontinuous with temperature. Liposome fusion could be detected within 35 ms and persisted for 180 min. Using liposomes containing defined Ca 2 + concentrations we have not found a major influence of Ca 2 + asymmetry on fusion. Incorporation of the ganglioside GM 3 , which is present in the membranes of intact adrenal medullary secretory vesicles did not change the properties of liposomes fusion. Using a Ca 2 +-selective electrode we have identified in secretory vesiele membranes both high affinity binding sites for Ca 2 + (/(} = 1.6 . 10-6 M) and low affinity sites (/(} = 1.2· 10-4 M). limited vesicles, which, upon stimulation during exocytosis, fuse with the cell membrane to release their (often macromolecular) content into the extracellular also fuse with each other resulting in 'compound exocytosis' [l]. In one of the first electron microscopical documents of the exocytotic process, Palade [2] has shown 'compound exocytosis' to occur in the exocrine pancreas. Subsequently, the observation of compound exocytosis has been extended to many other secretory cell types including adrenal medulla [3] [4] [5] [6] [7] [8] [9] [10] [11] . Based on these observations, we have used ,intervesicular fusion of isolated secretory vesicles as a model system for exocytosis, in order to obtain infor-0005-2736/81/0000~OOOOI$02.50
doi:10.1016/0005-2736(81)90265-0 pmid:6791690 fatcat:7bmkog4sgbfb7m6fwkocarj63a