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 (PubMed:27212239). Component of a ribosome-associated ER translocon complex involved in multi-pass membrane protein transport into the ER membrane and biogenesis (PubMed:32820719). 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 (PubMed:32820719). Homodimer and homotetramer (PubMed:27212239). Homodimer under resting conditions; forms homotetramers following and ER calcium overload (PubMed:27212239). 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 (PubMed:32820719). The first transmembrane region is required for localization to the endoplasmic reticulum (PubMed:27212239). A publication reported localization in cytoplasm and nucleus (PubMed:22714896). Nuclear localization is however in contradiction with two other reports (PubMed:10393320, PubMed:27212239). Widely expressed in adult and fetal tissues, with higher levels in thymus, prostate, testis and small intestine and lower levels in brain, placenta, lung and kidney (PubMed:10393320, PubMed:20018682). Present in most tissues in the eye, including the trabecular meshwork and retina (at protein level) (PubMed:22714896). The disease is caused by variants affecting the gene represented in this entry. Disease susceptibility is associated with variants affecting the gene represented in this entry. Belongs to the TMCO1 family.