Functional comparison of full-length palladin to isolated actin binding domain

Palladin is an actin binding protein that is specifically upregulated in metastatic cancer cells but also co-localizes with actin stress fibers in normal cells and is critical for embryonic development as well as wound healing. Of nine isoforms present in humans, only the 90 kDa isoform of palladin, comprising three immunoglobulin (Ig) domains and one proline-rich region, is ubiquitously expressed. Previous work has also established that the Ig3 domain of palladin is the minimal binding site for F-actin. In this work, we compare functions of the 90 kDa isoform of palladin to the isolated actin binding domain. To understand the mechanism of action for how palladin can influence actin assembly, we monitored F-actin binding and bundling as well as actin polymerization, depolymerization, and copolymerization. We also provide initial evidence that 90 kDa palladin exists in a closed conformation that prevents binding by the Ig3 domain to G-actin as compared to the isolated domain. Understanding the role of palladin in regulating the actin cytoskeleton may help us develop means to prevent cancer cells from reaching the metastatic stage of cancer progression.