Two Functionally Distinct Sources of Actin Monomers Supply the Leading Edge of Lamellipodia

• Leading-edge G-actin comes from recycled lamellipodia F-actin and a cytosolic pool
• Cytosolic G-actin requires thymosin β4 for optimal leading-edge localization
• Cytosolic G-actin is predominantly targeted to formins at the leading edge
• Thymosin β4 prevents G-actin from binding to Arp2/3 polymerization sites

SummaryLamellipodia, the sheet-like protrusions of motile cells, consist of networks of actin filaments (F-actin) regulated by the ordered assembly from and disassembly into actin monomers (G-actin). Traditionally, G-actin is thought to exist as a homogeneous pool. Here, we show that there are two functionally and molecularly distinct sources of G-actin that supply lamellipodial actin networks. G-actin originating from the cytosolic pool requires the monomer-binding protein thymosin β4 (Tβ4) for optimal leading-edge localization, is targeted to formins, and is responsible for creating an elevated G/F-actin ratio that promotes membrane protrusion. The second source of G-actin comes from recycled lamellipodia F-actin. Recycling occurs independently of Tβ4 and appears to regulate lamellipodia homeostasis. Tβ4-bound G-actin specifically localizes to the leading edge because it does not interact with Arp2/3-mediated polymerization sites found throughout the lamellipodia. These findings demonstrate that actin networks can be constructed from multiple sources of monomers with discrete spatiotemporal functions.

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