Analysis of the Arp2/3-independent activity of the WASp family protein Las17

The WASp family protein Las17 is the primary activator of the Arp2/3 complex in Saccharomyces cerevisiae and is essential for membrane invagination during endocytosis. Las17 has a similar domain structure to mammalian WASp/N-WASp with an N-terminal WH1 domain, a central proline rich region and a C-terminal WCA region. It was recently shown that the polyproline rich region of Las17 can bind to actin and promote actin filament formation in vitro. It has been hypothesised that at a site of endocytosis Las17 may provide the initial filaments necessary to allow the rapid burst of Arp2/3 mediated actin polymerisation required to achieve invagination. Proline tracts within the region were shown to influence this activity but the actin binding sites and mechanism of action were unclear. Over the course of this research we have identified the minimal component required for this Arp2/3-independent activity and characterised the key motifs involved. The effect of regions outside of this minimum component on actin polymerisation are probed and an exciting potential role in the direct regulation of filament elongation proposed. Mechanisms by which the Arp2/3 dependent and independent activities of Las17 may be regulated relative to each other are examined and the ability of the full-length protein to activate the Arp2/3 complex is found to be somewhat autoinhibited. This work provides further evidence for Las17 as a multifunctional regulator of the actin cytoskeleton in line with a growing body of work demonstrating that WASp family proteins are more than just direct activators of the Arp2/3 complex.