P400, inositol 1,4,5-trisphosphate receptor (InsP3-R), is a key protein to understanding the mechanisms of inositol 1,4,5-trisphosphate (InsP3)-mediated Ca2+ mobilization. We obtained the cerebellar-type P400/InsP3-R cDNA and generated an L cell transfectant (L15) that produces cDNA-derived P400/InsP3-R. In membranes, this protein displays high affinity, specificity, and capacity for InsP3, as does the cerebellar P400/InsP3-R. InsP3 can also induce greater 45Ca2+ release from the membrane vesicles of L15 cells than from those of control L cells. These results provide direct evidence that the cDNA-derived P400/InsP3-R protein is actually involved in physiological Ca2+ mobilization, through binding to InsP3 molecules in the same manner as the cerebellar P400/InsP3-R.