Leukotriene B4 (
LTB4) activates the
G-protein-coupled receptor leukotriene B4 receptor 1 (BLT1) to mediate a diverse array of cellular responses in leukocytes including chemotaxis,
calcium mobilization, degranulation, and gene expression. To determine the role of phosphorylation in BLT1 regulation, we generated mutants of BLT1 in which all of the
serine/
threonine residues in the C-tail are converted to
alanine or to
aspartate/
glutamate. These mutants expressed in rat basophilic
leukemia RBL-2H3 cells bound
LTB4 with similar affinity and activated all of the known functional activities of BLT1, albeit at different levels. The conversion of phosphorylation sites to
alanine resulted in enhanced
G-protein-mediated activities, whereas conversion to
aspartate/
glutamate resulted in reduced responses and a right shift in dose response, indicating that receptor phosphorylation is a critical regulator of
G-protein-mediated pathways. Surprisingly, translocation of
beta-arrestin and receptor internalization was completely independent of BLT1 phosphorylation. Real-time analysis of
beta-arrestin translocation and receptor internalization using digital fluorescence video microscopy in cells expressing a
red fluorescent protein labeled BLT1 and a
green fluorescent protein-tagged
beta-arrestin confirmed phosphorylation-independent
beta-arrestin translocation and internalization of BLT1. In
beta-arrestin-deficient mouse embryo fibroblasts, the BLT1 receptors failed to display endosomal localization upon stimulation. In these cells, co-expression of
beta-arrestin-
green fluorescent protein with BLT1-red fluorescent
protein resulted in co-localization of BLT1 and
beta-arrestin upon activation. Thus, receptor phosphorylation-dependent mechanisms regulate
G-protein-mediated pathways; however, phosphorylation-independent mechanisms regulate
beta-arrestin association and internalization of BLT1.