Calcium-selective channel required to prevent calcium stores from overfilling, thereby playing a key role in calcium homeostasis (PubMed:27212239). In response to endoplasmic reticulum (ER) overloading, assembles into a homotetramer, forming a functional calcium-selective channel, regulating the calcium content in endoplasmic reticulum store (By similarity). Necessary for the biogenesis and transport of multi-pass membrane proteins into the ER membrane (By similarity). Homodimer and homotetramer. Component of the ribosome-associated ER translocon complex. The first transmembrane region is required for localization to the endoplasmic reticulum. Belongs to the TMCO1 family.