CHARACTERIZATION OF THE G-PROTEIN-COUPLED RECEPTOR KINASE GRK4 - IDENTIFICATION OF 4 SPLICE VARIANTS

A novel human G protein-coupled receptor kinase was recently identifie d by positional cloning in the search for the Huntington's disease loc us (Ambrose, C., James, M., Barnes, G., Lin, C., Bates, G., Altherr, M ., Duyao, M., Groot, N., Church, D., Wasmuth, J. J., Lehrach, H., Hous man, D., Buckler, A., Gusella, J. F., and MacDonald, M. E. (1993) Hum. Mel. Genet. 1, 697-703). Comparison off the deduced amino acid sequen ce of GRK4 with those of the closely related GRK5 and GRK6 suggested t he apparent loss of 32 codons in the amino-terminal domain and 46 codo ns in the carboxyl terminal domain of GRK4. These two regions undergo alternative splicing in the GRK4 mRNA, resulting from the presence or absence of exons filling one or both of these apparent gaps. Each inse rted sequence maintains the open reading frame, and the deduced amino acid sequences are similar to corresponding regions of GRK5 and GRK6. Thus, the GRK4 mRNA and the GRK4 protein can exist as four distinct va riant forms. The human GRK4 gene is composed of 16 exons extending ove r 75 kilobase pairs of DNA. The two alternatively spliced exons corres pond to exons II and XV. The genomic organization of the GRK4 gene is completely distinct from that of the human GRK2 gene, highlighting the evolutionary distance since the divergence of these two genes. Human GRK4 mRNA is expressed highly only in testis, and both alternative exo ns are abundant in testis mRNA. The four GRK4 proteins have been expre ssed, and all incorporate [H-3]palmitate. GRK4 is capable of augmentin g the desensitization of the rat luteinizing hormone/chorionic gonadot ropin receptor upon coexpression in HEK293 cells and of phosphorylatin g the agonist-occupied, purified beta(3)-adrenergic receptor, indicati ng that GRK4 is a functional protein kinase.