G protein coupled receptors for C3a are expressed in human mast cell lines, differentiated CD34+ -derived primary human mast cells as well as skin mast cells. C3a induces Ca2+ mobilization, causes substantial degranulation and chemokine generation in human mast cells via the activation of Gi-family of G proteins. Removal of potential phosphorylation sites within the carboxyl terminus of C3aR leads to more robust degranulation when compared to wildtype receptors. These findings are consistent with the idea that, as in many other cell types, receptor phosphorylation desensitizes C3aR expressed in mast cells. Agonist occupied GPCRs are phosphorylated by a family of protein kinases, collectively known as G protein coupled receptor kinases. Of the seven known GRKs, four are expressed LY2835219 ubiquitously. It is well established that GPCR phosphorylation by GRKs leads to the recruitment of b-arrestin, which results in receptor desensitization and internalization. However, the role of specific GRKs on receptor regulation has only been appreciated recently. Studies with siRNA-mediated knockdown of GRKs in HEK293 cells have shown that agonist-induced phosphorylation of angiotensin II type 1A receptor and V2 vasopressin receptors are predominantly mediated by GRK2 and GRK3. Furthermore, knockdown of these GRKs attenuated both agonist-induced b-arrestin recruitment and receptor desensitization. In addition to desensitization, receptor phosphorylation by GRKs leads to the activation of extracellular signal-regulated kinases in a b-arrestin-dependent manner. In HEK293 cells, knockdown of GRK5 and GRK6 inhibits angiotensin II and vasopressin-induced b-arrestin-dependent ERK1/2 phosphorylation. These findings suggest that for angiotensin type IA and vasopressin receptors, agonist-induced receptor phosphorylation by GRK2/GRK3 leads to receptor desensitization but their phosphorylation by GRK5/GRK6 promotes b-arrestin-dependent ERK1/2 phosphorylation. However, for the chemokine receptor CXCR4, GRK2/GRK6 are involved in receptor desensitization whereas GRK3/GRK6 play an important role in positively regulating ERK1/2 activation. In transfected COS cells, overexpression of GRK2, GRK3, GRK5 or GRK6 results in enhancement of agonist-induced C3aR phosphorylation. Our previous studies in a transfected mast cell line, RBL-2H3 indicated that GRK2 may participate in C3aR desensitization. However, the roles of other GRKs on the regulation of receptor function in mast cells remain unknown.