Actin microfilaments facilitate the retrograde transport from the Golgi complex to the endoplasmic reticulum in mammalian cells

The morphology and subcellular positioning of the Golgi complex depend on b oth microtubule and actin cytoskeletons. In contrast to microtubules, the r ole of actin cytoskeleton in the secretory pathway in mammalian cells has n ot been clearly established. Using cytochalasin D, we have previously shown that microfilaments are not involved in the endoplasmic reticulum-Golgi me mbrane dynamics. However, it has been reported that, unlike botulinum C2 to xin and latrunculins, cytochalasin D does not produce net depolymerization of actin filaments. Therefore, we have reassessed the functional role of ac tin microfilaments in the early steps of the biosynthetic pathway using C2 toxin and latrunculin B. The anterograde endoplasmic reticulum-to-Golgi tra nsport monitored with the vesicular stomatitis virus-G protein remained una ltered in cells treated with cytochalasin D, latrunculin B or C2 toxin. Con versely, the brefeldin A-induced Golgi membrane fusion into the endoplasmic reticulum, the Golgi-to-endoplasmic reticulum transport of a Shiga toxin m utant form, and the subcellular distribution of the KDEL receptor were all impaired when actin microfilaments were depolymerized by latrunculin B or C 2 toxin. These findings, together with the fact that COPI-coated and uncoat ed vesicles contain beta/gamma -actin isoforms, indicate that actin microfi laments are involved in the endoplasmic reticulum/Golgi interface, facilita ting the retrograde Golgi-to-endoplasmic reticulum membrane transport, whic h could be mediated by the orchestrated movement of transport intermediates along microtubule and microfilament tracks.