WHIMP is a WASP-family Actin Nucleation-promoting Factor that Functions in Cell Motility

Actin filament assembly is regulated by many actin-binding proteins that function to maintain cell shape and structure, enable vesicle trafficking, control motility, and support DNA repair. Identifying these regulatory proteins is critical for a comprehensive understanding of the dynamic organization of actin in various cellular processes. In the last decade, the discovery of new Wiskott-Aldrich Syndrome Protein (WASP) family actin nucleation factors such as WASH, WHAMM, and JMY has significantly contributed to our understanding of actin cytoskeleton functions, and opened new avenues of research into the physiological roles of these proteins in membrane trafficking. I have identified a new WASP-family protein, WAVE Homology In Membrane Protrusions (WHIMP), which displays characteristic actin nucleation-promoting features. This study was aimed at determining the role of WHIMP in cells. My work shows that WHIMP is a weak activator of the Arp2/3 complex compared to other WASP-family members. Upon overexpression, WHIMP induces peripheral and dorsal membrane protrusions and co-localizes extensively with F-actin and the Arp2/3 complex at the edge of the cell. Moreover, migration assays show that WHIMP expression enhances cell motility by directly activating the Arp2/3 complex and inducing tyrosine phosphorylation and Src tyrosine kinase activation at the membrane ruffles. Rapid cell motility is abolished following WHIMP depletion, or upon expression of a WHIMP truncation mutant lacking its actin and the Arp2/3 binding region. My current findings provide insight into the actin assembly properties, expression, localization, and function of WHIMP as a new motility factor within the WASP family. Collectively, these findings identify a role for WHIMP in cell migration, as well as expand our understanding of proteins that regulate the Arp2/3 complex.