Involved in GABAergic but not glutamatergic transmission. Binds and traps GABAA receptors in the endoplasmic reticulum (ER). Modulates postsynaptic GABAergic transmission, and therefore inhibitory neurotransmission, by reducing the plasma membrane expression of these receptors. Altered GABAergic signaling is one among many causes of cleft palate (By similarity). Might function as a lipid scramblase, translocating lipids in membranes from one leaflet to the other one (By similarity). Required for efficient glycosylphosphatidylinositol (GPI) inositol deacylation in the ER, which is a crucial step to switch GPI-anchored proteins (GPI-APs) from protein folding to transport states (By similarity). May play a role in T-cell development (By similarity). Belongs to the CLPTM1 family.