UCSF

Most chemoattractants rely on activation of Gai to regulate directional cell migration, but few links from Gai to chemotactic effectors are known. Through affinity chromatography using primary neutrophil lysate, we identify Homer3 as a novel Gai2-binding protein. RNAi-mediated knockdown of Homer3 in neutrophil-like HL-60 cells impairs chemotaxis and the establishment of actin polarity. Most proteins that are required for cell polarity are needed for actin assembly or activation of core chemotactic effectors such as PI3K and Rac. In contrast, Homer3 knockdown cells show normal magnitude and kinetics of chemoattractant-induced activation of PI3K and Rac effectors as well as other downstream signals of Gai effectors such as calcium mobilization. Although chemoattractant-stimulated Homer3 knockdown cells exhibit normal initial actin assembly, they fail to polarize actin assembly and are defective at initiating cell polarity and motility. Our data suggests that Homer3 acts as a scaffold that spatially organizes actin assembly to support polarity and motility downstream of GPCR activation.