GPI-anchored cell surface protein that regulates T-lymphocytes proliferation, differentiation, and activation. Regulates the T-cell receptor (TCR signaling by interacting with component CD3Z/CD247 at the plasma membrane, leading to CD3Z/CD247 phosphorylation modulation (By similarity. Restricts the entry of human coronaviruses, including SARS-CoV, MERS-CoV and SARS-CoV-2, by interfering with spike protein-mediated membrane fusion. Plays also an essential role in placenta formation by acting as the main receptor for syncytin-A (SynA. Therefore, participates in the normal fusion of syncytiotrophoblast layer I (SynT-I and in the proper morphogenesis of both fetal and maternal vasculatures within the placenta. May also act as a modulator of nicotinic acetylcholine receptors (nAChRs activity (By similarity.