Abstract
Growing experimental evidence suggests that formin and Arp2/3 coexist with distinct molecular and functional properties in cell protrusions. However, current models of cell migration focus on Arp2/3-mediated actin assembly as the driver of lamellipodial protrusions. The roles of formins, and how formin-mediated filament assembly may be coordinated with Arp2/3-mediated assembly are unclear. Here, we investigated the roles of formin in cell migration. We found that mDia1 is present in lamellipodia of PtK1 cells. We then examined the recruitment dynamics of mDia1, Arp2/3, paxillin, and actin to the leading edge using quantitative live cell imaging. We found that mDia1, actin, and paxillin start to accumulate at maximum retraction edge velocity prior to the onset of protrusion events whereas Arp2/3 is recruited concomitantly with protrusions. After protrusions start, mDia1 and paxillin undergo transient decreases at maximum protrusion velocity while Arp2/3 and actin reach maximum with a delay of 10s. These results suggest that mDia1-mediated actin assembly initiates protrusions by promoting nascent adhesions, and fast protrusions are accompanied by transient decrease of adhesion formation and the reinforcement of actin polymerization by Arp2/3 against increasing membrane tension.
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