Architectural dynamics of F-actin in eupodia suggests their role in invasive locomotion in Dictyostelium

Eupodia are F-actin-containing cortical structures similar to vertebrate po dosomes or invadopodia found in metastatic cells. Eupodia are rich in cy-ac tinin and myosin IB/D, but not a Dictyostelium homologue of talin. In the p resent study, we localized other actin-binding proteins, ABP120, cofilin, c oronin, and fimbrin, in the eupodia and examined the three-dimensional orga nization of their F-actin system by confocal microscopy and transmission el ectron microscopy, To examine their function, we analyzed the assembly and disassembly dynamics of the F-actin system in eupodia and its relation to l amellipodial protrusion, Actin dynamics was examined by monitoring S65T-GFP -coronin and rhodamine-actin using a real-time confocal unit and a digital microscope system. Fluorescence morphometric analysis demonstrates the pres ence of a precise spatiotemporal coupling between F-actin assembly in eupod ia and lamellipodial protrusion. When a lamellipodium advances to invade a tight space, additional rows of eupodia are sequentially formed at the base of that lamellipodium. These results indicate that mechanical stress at th e leading edge modulates the structural integrity of actin and its binding proteins, such that eupodia are formed when anchorage is needed to boost fo r invasive protrusion of the leading edge. (C) 1999 Academic Press.