Figure 6.

Simplified Model Describing VILLIN-Mediated Regulation of Actin Dynamics at Pollen Tube Tips.

Actin filaments are nucleated by membrane-anchored formins within the apical dome of the pollen tube. Because formins bind the barbed end of actin filaments, we propose that the actin filaments are oriented with their pointed ends facing the cytoplasm. After nucleation, the actin filaments are instantly decorated with actin filament binding proteins, such as villin, which confer rigidity and allow the filaments to grow in a linear manner toward the cytoplasm. At the apex, membrane-anchored actin filaments can be turned over rapidly via severing (see 1 marked actin filaments), which is presumably mediated by villin acting in concert with the tip-focused calcium gradient. Some apex-originated actin filaments move to the apical flank (see 2 marked actin filaments), likely due to the fusion of exocytic vesicles at the apex of the pollen tube. The combination of these events leads to the outcome that actin filaments are less abundant at the pollen tube apex. In addition to actin filaments that move from the apex to the apical flank, actin filaments can also be nucleated at the membranes of the apical flank (see 3 marked actin filaments). Apical flank membrane-anchored actin filaments can be used for the construction of actin collars (see 4 marked actin filaments). In addition to conferring rigidity upon actin filaments and promoting linear growth toward the subapical cytoplasm, calcium-triggered, villin-mediated severing also promotes relocation of apical flank membrane-anchored actin filaments to actin collars (see 5 marked actin filament). Additionally, at the subapex, villin-mediated severing may facilitate the construction of actin collars by eliminating actin filaments that are not oriented longitudinally (see 6 marked actin filaments). The bundling activity of villin may also be directly involved in the construction of actin collars (see 7 marked actin filaments). Black arrow indicates pollen tube growth direction, and brown arrows indicate the direction of membrane flow.