Abstract

Dr Frank Lai Pui Ling

The assembly of actin filament is elegantly orchestrated by an array of regulatory molecules. Needless to say, this process is fundamental to many vital cellular functions, such as migration and endocytosis. In vivo actin filament assembly requires de novo actin nucleating factors, such as the Arp2/3 complex, which has emerged as a key regulator of many dynamic actin polymerisation events. The Arp2/3 complex mediated actin polymerisation is regulated, in turn, by cellular nucleation promoting factors (NPFs). Prominent members of NPFs include; the class I Scar/WAVE family proteins, such as WASp and WAVEs; and the class II cortactin family proteins. We employed advanced imaging techniques, such as Total Internal Refection Fluorescent (TIRF) microscopy, Fluorescence Recovery After Photobleaching (FRAP) and photo-activation in combination with genetic knockouts in mouse to study the molecular regulation of Arp2/3 dependent actin assembly in cells. Consistent with the “tip polymerisation model”, our dynamic turnover data on fluorescently tagged molecules defined the site of actin polymerisation and the Arp2/3 complex incorporation at the lamellipodium tip, coinciding with WAVE-complex assembly. We also revealed that, although WAVE proteins are important for cell migration, WASp proteins instead of WAVEs are more relevant in the regulation of receptor-mediated endocytosis. Surprisingly, however, our data suggest that the class II NPF, cortactin may not directly participate in nucleating actin filament but rather, may have a specific role in the regulation of signal transduction by small Rho-GTPases.

Host: Prof. Philipp Kaldis