| Abstract: | We investigated the effect of brefeldin A on membrane trafficking and the actin cytoskeleton of pollen tubes of Lilium longiflorum with fluorescent dyes, inhibitor experiments, and confocal laser scanning microscopy. The formation of a subapical brefeldin A-induced membrane aggregation (BIA) was associated with the formation of an actin basket from which filaments extended towards the tip. The orientation of these actin filaments correlated with the trajectories of membrane material stained by FM dyes, suggesting that the BIA-associated actin filaments are used as tracks for retrograde transport. Analysis of time series indicated that these tracks (actin filaments) were either stationary or glided along the plasma membrane towards the BIA together with the attached membranes or organelles. Disturbance of the actin cytoskeleton by cytochalasin D or latrunculin B caused immediate arrest of membrane trafficking, dissipation of the BIA and the BIA-associated actin basket, and reorganization into randomly oriented actin rods. Our observations suggest that brefeldin A causes ectopic activation of actin-nucleating proteins at the BIA, resulting in retrograde movement of membranes not only along but also together with actin filaments. We show further that subapical membrane aggregations and actin baskets supporting retrograde membrane flow can also be induced by calyculin A, indicating that dephosphorylation by type 2 protein phosphatases is required for proper formation of membrane coats and polar membrane trafficking. |
