Sequence Similarities between a Novel Putative G Protein-Coupled Receptor and Na+/Ca2+ Exchangers Define a Cation Binding Domain

Abstract

cDNA clones encoding a novel putative G protein-coupled receptor have been characterized. The receptor is widely expressed in normal solid tissues. Consisting of 1967 amino acid residues, this receptor is one of the largest known and is therefore referred to as a very large G protein-coupled receptor, or VLGR1. It is most closely related to the secretin family of G protein-coupled receptors based on similarity of the sequences of its transmembrane segments. As demonstrated by cell surface labeling with a biotin derivative, the recombinant protein is expressed on the surface of transfected mammalian cells. Whereas several other recently described receptors in this family also have large extracellular domains, the large extracellular domain of VLGR1 has a unique structure. It has nine imperfectly repeated units that are rich in acidic residues and are spaced at intervals of approximately 120 amino acid residues. These repeats resemble the regulatory domains of Na+/Ca2+ exchangers as well as a component of an extracellular aggregation factor of marine sponges. Bacterial fusion proteins containing two or four repeats specifically bind 45Ca in overlay experiments; binding is competed poorly by Mg2+ but competed well by neomycin, Al3+, and Gd3+. These results define a consensus cation binding motif employed in several widely divergent types of proteins. The ligand for VLGR1, its function, and the signaling pathway(s) it employs remain to be defined.

Very Large G Protein-coupled Receptor-1, the Largest Known Cell Surface Protein, Is Highly Expressed in the Developing Central Nervous System

“…In Situ Hybridization of VLGR1 to Mouse Embryos-A 1.6-kb mouse VLGR1 cDNA fragment was amplified from kidney total RNA using primers derived from mouse genomic DNA sequences; the cDNA was 80% identical to nucleotides 1957-3561 of the previously reported human VLGR1 open reading frame (7). A subclone was used as a probe for in situ hybridization with a panel of mouse embryo sections (Fig.…”

“…In all such receptors, the ectodomain is separated from the 7-TM domain by a G protein-coupled receptor proteolytic sequence at which the ectodomain can be specifically cleaved (18). Indeed, the ectodomain of VLGR1a can be cleaved when expressed in cultured cells, although the exact site of cleavage has not been determined (7).…”

“…We previously identified a putative member of this subfamily termed very large G protein-coupled receptor-1 (VLGR1) (7). Although the 7-TM domain of VLGR1 was related to those of Flamingo and latrophilin, its putative ectodomain was unique, consisting mostly of seven imperfect calcium exchanger ␤ (Calx-␤) repeats (8) resembling the calcium-binding regulatory domains of sodium-calcium exchangers.…”

“…Although the Calx-␤ repeats of VLGR1 indeed bound calcium (7), there were few additional clues as to the function of VLGR1. Its pattern of expression was uninformative, because VLGR1 was expressed at low levels in almost all adult human tissues examined.…”

The Very Large G-Protein-Coupled Receptor VLGR1: A Component of the Ankle Link Complex Required for the Normal Development of Auditory Hair Bundles

“…Vlgr1 is a member of the secretin family (family 2 or B) of G-protein-coupled receptors. It is distinguished by a very large ectodomain consisting mainly of calcium-binding Calx-␤ repeats that resemble the regulatory domains of sodium-calcium exchangers (Nikkila et al, 2000). The tertiary structure of these Calx-␤ repeats is probably calcium dependent, perhaps explaining why the ALA is sensitive to calcium chelation (Goodyear and Richardson, 1999).…”