Polytopic transmembrane protein. Inhibits regulated intramembrane proteolysis (RIP) of CREB3L1, inhibiting its activation and the induction of collagen synthesis. In response to ceramide, which alters TM4SF20 membrane topology, stimulates RIP activation of CREB3L1. Ceramide reverses the direction through which transmembrane helices are translocated into the endoplasmic reticulum membrane during translation of TM4SF20, this mechanism is called 'regulated alternative translocation' (RAT) and regulates the function of the transmembrane protein. Ceramide alters the direction through which transmembrane helices are translocated into the endoplasmic reticulum membrane during translation of TM4SF20. The first transmembrane helix plays a critical role for the insertion orientation in the endoplasmic reticulum membrane. Glycosylated at Asn-132, Asn-148 and Asn-163 in presence of ceramide which inverts the orientation of TM4SF20 in membranes exposing these residues to the endoplasmic reticulum lumen. Cleaved by signal peptidase at Ser-14 but the peptide does not act as a signal peptide. Cleavage is inhibited by ceramide which inverts the orientation of TM4SF20 in membranes exposing the N-terminus to the cytosol and not to the endoplasmic reticulum lumen. Belongs to the L6 tetraspanin family.