Human Trp3 forms both inositol trisphosphate receptor-dependent and receptor-independent store-operated cation channels in DT40 avian B lymphocytes

Mammalian Trp proteins are candidates for plasma membrane calcium channels regulated by receptor activation or by intracellular calcium store depletion [capacitative calcium entry (CCE)]. One extensively investigated member of the Trp family, the human Trp3 (hTrp3), behaves as a receptor-activated, calcium-permeable, nonselective cation channel when expressed in cell lines and does not appear to be activated by store depletion. Nonetheless, there is good evidence that Trp3 can be regulated by interacting with inositol trisphosphate receptors (IP(3)Rs), reminiscent of the conformational coupling mode of CCE. To investigate the role of Trp3 in CCE, and its regulation by IP3R, we transiently expressed hTrp3 in the wild-type DT40 chicken B lymphocyte cell line and its variant lacking IP3R. Expression of hTrp3 in either wild-type or IP3R-knockout cells did not increase basal membrane permeability, but resulted in a substantially greater divalent cation entry after thapsigargin-induced store depletion. This hTrp3-dependent divalent cation entry was significantly greater in the wild type than in IP3R-knockout cells. Thus, it appears that in this cell line, hTrp3 forms channels that are store-operated by both IP3R-dependent and IP3R-independent mechanisms. Trp3, or one of its structural relatives, is a candidate for the store-operated, nonselective cation channels observed in smooth muscle cells and other cell types.