Association of Cdc42/N‐WASP/Arp2/3 Signaling Pathway with Golgi Membranes

Recent findings indicate that Cdc42 regulates Golgi‐to‐ER (endoplasmic reticulum) protein transport through N‐WASP and Arp2/3 (Luna et al. 2002, Mol. Biol. Cell, 13:866–879). To analyse the components of the Cdc42‐governed signaling pathway in the secretory pathway, we localized Cdc42, N‐WASP and Arp2/3 in the Golgi complex by cryoimmunoelectron microscopy. Cdc42 is found throughout the Golgi stack, particularly in cis/middle cisternae, whereas N‐WASP and Arp3 (a component of the Arp2/3 complex) are restricted to cis cisternae. Arp3 also colocalized in peri‐Golgi tubulovesicular structures with either KDEL receptor or GM130. Even though Arp3 is not found in TGN46‐positive cisternal elements, a small fraction of Arp3‐labeled tubulo‐vesicular elements showed TGN46 labeling. Active Cdc42 (GTP‐bound form) induced relocation of N‐WASP and Arp3 to the lateral rims of Golgi cisternae. These results show that the actin nucleation and polymerization signaling pathway governed by Cdc42/N‐WASP/Arp operates in the Golgi complex of mammalian cells, further implicating actin dynamics in Golgi‐associated membrane trafficking.