Palladin nucleates actin assembly and regulates cytoskeleton architecture
Palladin is an actin crosslinking protein that uses an immunoglobulin (Ig) domain to bind F-actin. Expression of palladin correlates with increased cell motility in normal cells during development and wound healing, but also correlates with the invasive motility of abnormal cells such as those involved in metastasis. In particular, the correlation between the loss of palladin and decreased levels of actin polymer suggests that palladin may have a direct role in stabilizing F-actin and/or enhancing actin polymerization. While palladin has been causally linked to the invasive cell motility associated with metastasis, the mechanistic roles of palladin in organizing cellular actin networks and governing actin filament dynamics have remained unclear. Here we show that the actin binding domain of palladin (designated as Palld-Ig3 from here on) increases the rate of actin polymerization in vitro via a mechanism that involves filament nucleation and elongation. While Palld-Ig3 does not alter actin critical concentration, it does modestly enhance the rate of filament elongation. The major effect of Palld-Ig3 in stimulating actin filament formation is due to an increase in nucleation rate. The filaments nucleated by Palld-Ig3 domain are also highly crosslinked. Our results suggest dual roles for Palld-Ig3 that includes both promoting actin polymerization and modifying the stability of actin filaments. These roles provide a possible mechanistic explanation for palladin's critical in vivo functions in generating actin filament structures required for normal cell adhesion as well as cell motility associated with cancer metastasis.
Thesis (Ph.D.)-- Wichita State University, Fairmount College of Liberal Arts and Sciences, Dept. of Chemistry