We several projects. 1. Diverse chemokines bind to G protein-coupled receptors (GPCRs) to activate the small GTPase Rac to regulate F-actin dynamics during chemotaxis. ELMO and Dock proteins form complexes that function as guanine nucleotide exchange factors (GEFs) for Rac activation. However, the linkage between GPCR activation and the ELMO/Dock-mediated Rac activation is not fully understood. In the present study, we show that chemoattractants induce dynamic membrane translocation of ELMO1 in mammalian cells. ELMO1 plays an important role in GPCR-mediated chemotaxis. We also reveal that ELMO1 and Dock1 form a stable complex. Importantly, activation of chemokine GPCR promotes the interaction between ELMO1 and G. The ELMO1-G interaction is through the N-terminus of ELMO1 protein and is important for the membrane translocation of ELMO1. ELMO1 is required for Rac1 activation upon chemoattractant stimulation. Our results suggest that chemokine GPCR-mediated interaction between G and ELMO1/Dock1 complex might serve as an evolutionarily conserved mechanism for Rac activation to regulate actin cytoskeleton for chemotaxis of human cells (Wang et a;. J of Cancer, 2016). 2. What is the role of beta-arrestin in chemokine GPCR-mediated neutrophil chemotaxis? -arrestins have emerged as key regulators of cytoskeletal re-arrangement that are required for directed cell migration. While it is known that -arrestins are required for formyl-Met-Leu-Phe receptor (FPR) recycling, less is known about their role in regulating FPR mediated neutrophil chemotaxis. Here, we show that -arrestin1 (ArrB1) co-localized with F-actin in the leading edge of neutrophil-like HL-60 cells during chemotaxis and its knockdown resulted in markedly reduced migration within fMLP gradients. The small GTPase Rap2 was found to bind ArrB1 under resting conditions but dissociated upon fMLP stimulation. The FPR-dependent activation of Rap2 required ArrB1 but was independent of Gi activity. Significantly, depletion of either ArrB1 or Rap2 resulted in reduced chemotaxis and defects in cellular re-polarization within fMLP gradients. These data strongly suggest a model in which FPR is able to direct ArrB1 and other bound proteins that are required for lamillopodial extension to the leading edge in migrating neutrophils, thereby orientating and directing cell migration (Gera et al, J of Leukocyte Biol. in press).
