Control of local actin assembly by membrane fusion-dependent compartment mixing

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Local actin assembly is associated with sites of exocytosis in processes ranging from phagocytosis to compensatory endocytosis. Here, we examine whether the trigger for actin-coat assembly around exocytosing Xenopus egg cortical granules is 'compartment mixing' — the union of the contents of the plasma membrane with that of the secretory granule membrane. Consistent with this model, compartment mixing occurs on cortical granule–plasma membrane fusion and is required for actin assembly. Compartment mixing triggers actin assembly, at least in part, through diacylglycerol (DAG), which incorporates into the cortical granule membranes from the plasma membrane after cortical granule–plasma membrane fusion. DAG, in turn, directs long-term recruitment of protein kinase Cβ (PKCβ) to exocytosing cortical granules, where it is required for activation of Cdc42 localized on the cortical granules. The results demonstrate that mixing of two membrane compartments can direct local actin assembly and indicate that this process is harnessed during Xenopus egg cortical granule exocytosis to drive compensatory endocytosis.

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Figure 1: Actin-coat assembly occurs after plasma membrane and cortical granule membrane incorporation on cortical granule exocytosis.
Figure 2: Ca2+ elevation triggers DAG generation on the plasma membrane, which incorporates into the cortical granule membrane on cortical granule–plasma membrane fusion.
Figure 3: DAG incorporation into cortical granule membranes occurs before actin assembly and is dependent on cortical granule–plasma membrane compartment mixing.
Figure 4: DAG is required for actin-coat assembly.
Figure 5: PKCβ is transiently recruited to all cortical granules on Ca2+ elevation, but only remains on those that have exocytosed.
Figure 6: Perturbation of PKCβ affects actin-coat assembly.
Figure 7: Perturbation of PKCβ affects activation of Cdc42, which is localized on cortical granule membranes before egg activation.
Figure 8: Schematic representation of how compartment mixing results in actin-coat assembly.


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We thank B. Burkel for the Utr1–261–mRFP clone, M. Danilchik for the F-eGFP clone, T. Gomez for the Myc–Cdc42 clone, M. Kirschner for the Toca-1 antibody, T. Martin for providing BotA, and R. Tsien for the mRFP clone. This work is supported by National Institutes of Health (grant GM52932) to W.M.B.

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Correspondence to Hoi-Ying E. Yu.

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