Interacts with target proteins during their translocation into the lumen of the endoplasmic reticulum. Protects unfolded target proteins against degradation during ER stress. May facilitate glycosylation of target proteins after termination of ER stress. May modulate the use of N-glycosylation sites on target proteins (By similarity). Interacts with SEC61B, SEC61A1 and the SEC61 complex. Interacts with CANX (By similarity). Belongs to the RAMP4 family.