The Sec/MUNC18 (SM) protein MUNC18-1 regulates the secretory pathway and binds the exocytosis protein syntaxin. Mouse chromaffin cells (MCCs) without MUNC18-1 exhibit impaired exocytosis and defective syntaxin-1 (STX1) targeting, but also an increase in cortical F-actin. In their Research Article, Matthijs Verhage and colleagues (Pons-Vizcarra et al., 2019) addressed the mechanism of cortical F-actin accumulation, showing that MUNC18-1 independently regulates F-actin and STX1 targeting. By screening different mutations of MUNC18-1, the authors found one mutation that differentiates between these two cellular functions of MUNC18-1: the MUNC18-1(V263T) mutant protein restored the correct localisation of STX1 at the membrane, but not a normal F-actin distribution when expressed in MUNC18-1-knockout MCCs. The authors noticed that its two paralogs, MUNC18-2 and MUNC-3, do not have a hydrophobic residue at this position, and found that expression of these paralogs also failed to restore normal F-actin accumulation. The authors then replaced the equivalent non-hydrophobic residue in MUNC18-2 with a hydrophobic valine residue, and observed a normal F-actin distribution, thus attributing the hydrophobic V263 in MUNC18-1 as regulating F-actin accumulation, but not STX1 targeting. This study provides a separation of function of a key factor for the secretory pathway and conclusively shows that cortical F-actin does not limit the docking and fusion of secretory vesicles.

• © 2019. Published by The Company of Biologists Ltd