Agonist-activated Ca2+ influx occurs at stable plasma membrane and endoplasmic reticulum junctions.

Junctate is a 33 kDa integral protein of sarco(endo)plasmic reticulum membranes that forms a macromolecular complex with inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] receptors and TRPC3 channels. TIRF microscopy shows that junctate enhances the number of fluorescent puncta on the plasma membrane. The size and distribution of these puncta are not affected by the addition of agonists that mobilize Ca(2+) from Ins(1,4,5)P(3)-sensitive stores.