Calcium-selective channel required to prevent calcium stores from overfilling, thereby playing a key role in calcium homeostasis. 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. Component of a ribosome-associated ER translocon complex involved in multi-pass membrane protein transport into the ER membrane and biogenesis. Together with SEC61 and TMEM147, forms the lipid-filled cavity at the center of the translocon where TMEM147 may insert hydrophobic segments of mutli-pass membrane proteins from the lumen into de central membrane cavity in a process gated by SEC61, and TMCO1 may insert hydrophobic segments of nascent chains from the cytosol into the cavity. Homodimer and homotetramer. Homodimer under resting conditions; forms homotetramers following and ER calcium overload. The ribosome-associated ER translocon complex includes SEC61A1, SEC61B, SEC61G, TMCO1, CCDC47, NCLN/Nicalin, NOMO and TMEM147; in the absence of ribosomes, only the complex forms with NCLN/Nicalin, NOMO and TMEM147 remains intact. The first transmembrane region is required for localization to the endoplasmic reticulum. Widely expressed, with high levels in liver, kidney and heart, intermediate amounts in back fat, longissimus dorsi, brain, lymph, large intestine, small intestine and lung, and low levels in spleen and stomach. Belongs to the TMCO1 family.