Cloning of a Molluscan G Protein alpha Subunit of the Gq Class which is Expressed Differentially in Identified Neurons

Through molecular cloning we have identified a molluscan G protein alpha subunit which belongs to the G alpha q family and is expressed in the central nervous system (CNS) of the pond snail, Lymnaea stagnalis. The deduced protein product shares a very high degree of amino sequence identity with vertebrate and invertebrate G alpha q/G alpha 11 subunits (80-82% and 76-77%, respectively). Large parts of the protein have been completely conserved, among which are residues 25-58, including the nucleotide-binding A domain. Especially the C-terminal half (amino acids 195-353), implicated in receptor and effector interactions, is highly conserved (94% sequence identity with murine sequences). This region includes the nucleotide-binding C, G, and I domains, which are identical to cognate motifs of vertebrate G alpha q/11. Like the latter proteins, the Lymnaea G alpha q C-terminus lacks a cysteine that could serve as a substrate for pertussis toxin. In situ hybridization reveals G alpha q-encoding mRNA(s) to be present throughout the CNS. Interestingly, however, close inspection of two identified cell types in the cerebral ganglia, the light-green cells, involved in the regulation of growth and metabolism and the anterior lobe cells which are involved in the control of male aspects of reproduction, indicates that they express the mRNA(s) at significantly different levels. Even within the heterologous cluster of light-green cells there appears to be differential expression of the pertinent mRNA. Such observations have hitherto not been reported for specific cell types occurring in vivo.