A novel G protein α subunit in embryo of the ascidian, Halocynthia roretzi

A cDNA clone encoding a novel G protein α subunit, HrGα_n was isolated from the larvae of ascidian, Halocynthia roretzi. In contrast with overall amino acid identity (63%) with G protein α subunit of G_i or G_o subclass, HrGα_n has a unique amino acid sequence, which lacks a residue for pertussis toxin substrate, but retains for cholera toxin substrate for ADP-ribosylation. The sequence characteristics and molecular phylogenetic analysis suggest that HrGα_n defines a novel subclass within G_i class of G protein α subunits. The zygotic expression of HrGα_n was first detected at the 64-cell stage and observed in all blastomeres except for B7.4, B7.5 and B7.6 cells till the 110-cell stage. As progress of the developmental stages, the expression of HrGα_n became restricted and was observed in the muscle, mesenchyme and a part of trunk lateral cells in tailbud embryos. With HrGα_n-GFP fusion-gene construct it was showed that the genomic fragment containing 2674 bp upstream of the putative translation start site of HrGα_n contained the regulatory sequence responsible for the expression in the muscle and mesenchyme cells, and that the regulatory sequence functioned also in Ciona intestinalis. Our results suggest a possible involvement of HrGα_n in the signaling system regulates the cell fate during the embryogenesis of the ascidian.