The broad goal of research in my lab is to define the function and regulation of heterotrimeric G proteins in leukocyte migration and tumor metastasis. The major focus of our research is to elucidate how G protein βγ subunits orchestrate the formation of specific signaling complexes to promote directional cell migration. Particularly, we are interested in understanding how Gβγ functions are regulated by a group of novel interacting proteins, WD40 repeat-containing proteins, which we recently identified by a yeast two-hybrid screen.

Free Gβγ liberated from the activated Gi/o family of G proteins is a master regulator of signal transduction pathways that control chemokine-induced chemotaxis of diverse cells, ranging from neurons, leukocytes to tumor cells. Over the last decade, significant progress has been made in understanding how Gβγ stimulates key signaling molecules such as PI3Kγ to promote cell migration. However, Gβγ has diverse interacting proteins. How it coordinates the activation of various effectors in the highly specific temporal and spatial manner required for precise control of cell migration remains elusive.

Our recent identification of WD40 repeat-containing proteins as novel Gβγ- interacting proteins opens a new era in understanding the function and regulation of Gβγ signaling. These proteins are predicted to form a similar β-propeller structure with multiple surfaces as Gβ (Figure). Therefore, like Gβγ, they have the potential to assemble wide array of proteins to promote or inhibit Gβγ-mediated signal transduction. Our studies from one of these proteins, receptor for activated C kinase 1 (RACK1), have yielded significant insights into the molecular basis for the interactions of WD40 repeat proteins with Gβγ, and the role of these proteins in regulating Gβγ-mediated cell migration. RACK1 is found to bind to a unique side-surface of Gβγ through a novel protein-protein interaction, WD40-WD40 repeat interaction. Binding of RACK1 to Gβγ selectively abrogates the activation of key signaling molecules, PI3K and PLCβ, resulting in inhibition of leukocyte migration. RACK1 therefore may constitute an important negative regulator that controls the amplitude of leukocyte migration. Stemming from these exciting findings, our current research interests include the following areas:

To investigate specific roles of RACK1-mediated negative regulation of leukocyte migration in physiological and pathological processes of immune responses; To determine the function of RACK1/ Gβγ interaction in tumor metastasis, since chemokine receptors, which are frequently found to be overexpressed in tumor, also promote tumor cell migration through Gβγ; To delineate the function of other Gβγ-interacting WD40 repeat proteins in leukocyte migration and tumor metastasis; To understand the structural aspect of the WD40-WD40 repeat interaction by x-ray crystallography.

To accomplish the outlined projects, we are using a combination of molecular and cellular techniques, animal models, fluorescence-based and FRET-based biophysical approaches. Moreover, we are using live cell imaging to monitor the complex and vivid process of cell migration. These studies could give rise to new insight into the signaling mechanisms governing leukocyte migration and tumor metastasis as well as uncovering new therapeutic targets.