Non-full-length Water-Soluble CXCR4QTY and CCR5QTY Chemokine Receptors: Implication for Overlooked Truncated but Functional Membrane Receptors

It was posited that functionalities of GPCRs require full-length sequences that are negated by residue deletions. Here we report that significantly truncated nfCCR5(QTY) and nfCXCR4(QTY) still bind native ligands. Receptor-ligand interactions were discovered from yeast 2-hybrid screening and confirmed by mating selection. Two nfCCR5(QTY) (SZ218a, SZ190b) and two nfCXCR4(QTY) (SZ158a, SZ146a) were expressed in E. coli. Synthesized receptors exhibited alpha-helical structures and bound respective ligands with reduced affinities. SZ190b and SZ158a were reconverted into non-QTY forms and expressed in HEK293T cells. Reconverted receptors localized on cell membranes and functioned as negative regulators for ligand-induced signaling when co-expressed with full-length receptors. CCR5-SZ190b individually can perform signaling at a reduced level with higher ligand concentration. Our findings provide insight into essential structural components for CCR5 and CXCR4 functionality, while raising the possibility that non-full-length receptors may be resulted from alternative splicing and that pseudo-genes in genomes may be present and functional in living organisms.