Expression of truncated transient receptor potential protein 1α (Trp1α). Evidence that the Trp1 C terminus modulates store-operated Ca²⁺ entry

Transient receptor potential protein 1 (Trp1) has been proposed as a component of the store-operated Ca²⁺ entry (SOCE) channel. However, the exact mechanism by which Trp1 is regulated by store depletion is not known. Here, we examined the role of the Trp1 C-terminal domain in SOCE by expressing hTrp1α lacking amino acids 664-793 (ΔTrp1α) or full-length hTrp1α in the HSG (human submandibular gland) cell line. Both carbachol (CCh) and thapsigargin (Tg) activated susrained Ca²⁺ influx in control (nontransfected), ΔTrp1α-, and Trp1α-expressing cells. Sustained [Ca²⁺](i), following stimulation with either Tg or CCh in ΔTrp1α-expressing cells, was about 1.5-2-fold higher than in Trp1α-expressing cells and 4-fold higher than in control cells. Importantly, (i) basal Ca²⁺ influx and (ii) Tg- or CCh-stimulated internal Ca²⁺ release were similar in all the cells. A similar increase in Tg-stimulated Ca²⁺ influx was seen in cells expressing Δ2Trp1α, lacking the C-terminal domain amino acid 649-793, which includes the EWKFAR sequence. Further, both inositol 1,4,5-trisphosphate receptor-3 and caveolin-1 were immunoprecipitated with ΔTrp1α and Trp1α. In aggregate, these data suggest that (i) the EWKFAR sequence does not contribute significantly to the Trp1-associated increase in SOCE, and (ii) the Trp1 C-terminal region, amino acids 664-793, is involved in the modulation of SOCE.