TY - JOUR
T1 - Actin filament disassembly is a sufficient final trigger for exocytosis in nonexcitable cells
AU - Muallem, Shmuel
AU - Kwiatkowska, Katarzyna
AU - Xu, Xin
AU - Yin, Helen L.
PY - 1995/2
Y1 - 1995/2
N2 - Although the actin cytoskeleton has been implicated in vesicle trafficking, docking and fusion, its site of action and relation to the Ca2+-mediated activation of the docking and fusion machinery have not been elucidated. In this study, we examined the role of actin filaments in regulated exocytosis by introducing highly specific actin monomer-binding proteins, the β-thymosins or a gelsolin fragment, into streptolysin O- permeabilized pancreatic acinar cells. These proteins had stimulatory and inhibitory effects. Low concentrations elicited rapid and robust exocytosis with a profile comparable to the initial phase of regulated exocytosis, but without raising [Ca2+], and even when [Ca2+] was clamped at low levels by EGTA. No additional cofactors were required. Direct visualization and quantitation of actin filaments showed that β-thymosin, like agonists, induced actin depolymerization at the apical membrane where exocytosis occurs. Blocking actin depolymerization by phalloidin or neutralizing β- thymosin by complexing with exogenous actin prevented exocytosis. These findings show that the conical actin network acts as a dominant negative clamp which blocks constitutive exocytosis. In addition, actin filaments also have a positive role. High concentrations of the actin depolymerizing proteins inhibited all phases of exocytosis. The inhibition overrides stimulation by agonists and all downstream effectors tested, suggesting that exocytosis cannot occur without a minimal actin cytoskeletal structure.
AB - Although the actin cytoskeleton has been implicated in vesicle trafficking, docking and fusion, its site of action and relation to the Ca2+-mediated activation of the docking and fusion machinery have not been elucidated. In this study, we examined the role of actin filaments in regulated exocytosis by introducing highly specific actin monomer-binding proteins, the β-thymosins or a gelsolin fragment, into streptolysin O- permeabilized pancreatic acinar cells. These proteins had stimulatory and inhibitory effects. Low concentrations elicited rapid and robust exocytosis with a profile comparable to the initial phase of regulated exocytosis, but without raising [Ca2+], and even when [Ca2+] was clamped at low levels by EGTA. No additional cofactors were required. Direct visualization and quantitation of actin filaments showed that β-thymosin, like agonists, induced actin depolymerization at the apical membrane where exocytosis occurs. Blocking actin depolymerization by phalloidin or neutralizing β- thymosin by complexing with exogenous actin prevented exocytosis. These findings show that the conical actin network acts as a dominant negative clamp which blocks constitutive exocytosis. In addition, actin filaments also have a positive role. High concentrations of the actin depolymerizing proteins inhibited all phases of exocytosis. The inhibition overrides stimulation by agonists and all downstream effectors tested, suggesting that exocytosis cannot occur without a minimal actin cytoskeletal structure.
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U2 - 10.1083/jcb.128.4.589
DO - 10.1083/jcb.128.4.589
M3 - Article
C2 - 7860632
AN - SCOPUS:0028889137
SN - 0021-9525
VL - 128
SP - 589
EP - 598
JO - Journal of Cell Biology
JF - Journal of Cell Biology
IS - 4
ER -