Actin assembly induced by polylysine beads or purified phagosomes: quantitation by a new flow cytometry assay

Actin assembly on biological membranes is a poorly understood process. We have previously shown that phagosomal membranes could induce actin assembly in the presence of thymosin beta4 (an actin sequestering protein that inhibits nonspecific nucleation), via the barbed ends of actin filaments.

Here, we have developed an in vitro system based on fluorescein-labeled G (monomeric) actin and flow cytometry analysis, which allowed us to quantify de novo actin assembly on the cytoplasmic side of purified phagosomes. To standardize the system, we also used latex beads covalently coupled with polylysine, which efficiently promote actin nucleation.

Flow cytometry analysis showed that the percentage of polylysine beads positive for F-actin filaments increased in a time- and G-actin concentration-dependent manner. Incubation of phagosomes with reagents affecting actin dynamics allowed us to extend our previous data showing that the phagosomal membranes assemble actin filaments de novo. Finally, our results pin-point a potential role for gelsolin as a positive regulator of actin assembly on the phagosomal membrane.

We propose that our system could facilitate the development of other in vitro assays for the analysis of actin assembly and its links to signaling in cells.