We isolated a cDNA encoding a novel protein with GTP-binding motifs from the salmon brain. The cloned 5.5-kb cDNA encoded 971 amino acids, which showed limited homology with members of large GTP-binding proteins, such as dynamin, MX protein and VPS1. By in vitro translation analysis, the size of the encoded protein was estimated to be 120 kDa. The recombinant protein including the putative GTP-binding domain was shown to bind to GTP in vitro. mRNA was strongly expressed in the brain, ovary and skeletal muscle. In the brain, expression of the mRNA was observed specifically in motor neurons, in nucleus oculomotorius, in nucleus valvulae lateralis, in the medulla oblongata and in the spinal cord. To determine the cell biological significance of the encoded protein, we transfected CHO cells with wild type or a putative constitutive active mutant of the mG120 (120-kDa G protein expressed in motor neurons) and examined morphological changes. However, no clear changes were observed. The functional role of this novel motor neuron-specific large GTP-binding protein remains to be elucidated.