ABSTRACT
C5a anaphylatoxin, a potent inflammatory mediator, is known to act through a specific G protein coupled receptor. However, some of the complex effects of C5a in vivo may not be explained solely by the deletion of the known receptor. Here, we show that an orphan receptor, identified as C5L2, is a high affinity C5a binding protein. Unlike the previously described C5aR, C5L2 is obligately uncoupled from heterotrimeric G proteins, in part by virtue of an amino acid alteration in the so-called DRY sequence at the end of the third transmembrane segment. Both human and murine C5L2 bear a leucine for arginine replacement at this site. C5L2, when transfected into several cell types, is weakly phosphorylated in transfected cells following binding of C5a but does not induce significant activation of MAP kinases, mediate calcium flux, or stimulate chemotaxis. Bone marrow cells from wild type respond robustly to C5a with induction and suppression of a number of inflammation related genes. In contrast, C5a receptor deficient mice, which bear C5L2 alone, do not respond to C5a with changes in gene transcription by microarray analyses. Biophysical properties of the C5L2, including slow ligand on and off rates, absence of internalization, and relatively high affinity for the C5a des Arg metabolite, suggest that this receptor may serve to modulate C5a biological functions in vivo. Finally, in contrast to previous reports, we find absolutely no interaction of C5L2 with other anaphylatoxins C3a and C4a.
Subject(s)
Complement C5a, des-Arginine/chemistry , Complement C5a/metabolism , Membrane Proteins , Receptors, Chemokine/metabolism , Amino Acid Sequence , Animals , Antigens, CD/chemistry , Antigens, CD/metabolism , Binding Sites , Binding, Competitive , Blotting, Western , Bone Marrow Cells/metabolism , Calcium/metabolism , Cloning, Molecular , Complement C5a, des-Arginine/metabolism , GTP-Binding Proteins/metabolism , Gene Expression Profiling , Humans , Inhibitory Concentration 50 , Ligands , MAP Kinase Signaling System , Mice , Mice, Knockout , Molecular Sequence Data , Oligonucleotide Array Sequence Analysis , Phosphorylation , Protein Binding , Receptor, Anaphylatoxin C5a , Receptors, Complement/metabolism , Recombinant Proteins/metabolism , Sequence Homology, Amino Acid , Signal Transduction , Transfection , Tumor Cells, CulturedABSTRACT
In this paper, we describe an assay using radioactive rubidium (86Rb) efflux to screen functional human ether-a go-go-related gene (HERG) K+ channels in a high-throughput screening (HTS) format. This assay offers an alternative way to examine junctional interactions between chemical compounds and HERG K+ channels. Follow-up experiments and discussions were carried out to address a variety of factors that affect potency evaluation within the Rb efflux assay. Factors that can affect the assay results, such as assay time, efflux rate, and compound blocking kinetics, are discussed in detail. Our results provide some explanations for the variances of the assay results and offer some guidelines for using the Rb efflux assay to evaluate compound interactions with HERG K+ channels in the pharmaceutical industry.
