Differential distribution of the G protein γ3 subunit in the developing zebrafish nervous system

Abstract G proteins play an essential role in the transduction and propagation of extracellular signals across the plasma membrane. It was once thought that the G protein α subunit was the sole regulator of intracellular molecules. The G protein βγ complex is now recognized as participating in many signaling events. While screening a zebrafish cDNA library to identify members of the protein 4.1 superfamily (Kelly, G.M., Reversade, B., Biochem. Cell Biol. 75 (1997), 623), we fortuitously identified a clone that encodes a zebrafish G protein γ subunit. The 666 nucleotides of the zebrafish G protein γ subunit cDNA encodes a polypeptide of 75 amino acids with high degree of homology to human, bovine, rat and mouse γ subunits. BLAST search analysis of GenBank revealed that the zebrafish γ subunit is 93% identical and 97% similar to the mammalian γ3 subunit. The γ3 gene was mapped to the zebrafish linkage group 21, approximately 10.76 cRays from bf , a gene with sequence homology to the human properdin factor gene. RT-PCR and in situ hybridization analyses first detected γ3 mRNA during late somitogenesis, where it was expressed preferentially in the Vth cranial nerve, the forebrain and in ventrolateral regions of the mid- and hindbrain including the spinal cord. The ability of the zebrafish γ3 subunit to form a signaling heterodimeric complex with a β subunit was tested using a human β2 subunit. The γ3 formed a heterodimer with β2 and the complex was capable of binding calmodulin in a calcium-dependent manner. Overexpression of the β2γ3 complex in zebrafish embryos lead to the loss of dorsoanterior structures and heart defects, possibly owing to an up-regulation of mitogen-activated protein kinase activity and/or decline in protein kinase A signaling. Together, these data imply that a βγ heterodimer plays a role in signal transduction events involving G protein coupled receptors and that these events occur in specific regions in the nervous system of the developing zebrafish.