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

Abstract

A cDNA clone encoding a novel G protein α subunit, HrGαn was isolated from the larvae of ascidian, Halocynthia roretzi. In contrast with overall amino acid identity (63%) with G protein α subunit of Gi or Go subclass, HrGαn 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 HrGαn defines a novel subclass within Gi class of G protein α subunits. The zygotic expression of HrGαn 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 HrGαn became restricted and was observed in the muscle, mesenchyme and a part of trunk lateral cells in tailbud embryos. With HrGαn-GFP fusion-gene construct it was showed that the genomic fragment containing 2674 bp upstream of the putative translation start site of HrGαn 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 HrGαn in the signaling system regulates the cell fate during the embryogenesis of the ascidian.