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. Interacts with SEC61B, SEC61A1 and the SEC61 complex. Interacts with CANX. Detected in astrocytes, brain, lung, liver, kidney, spleen and testis. Up-regulated in cultured astrocytes in response to hypoxia; levels remain elevated for at least 4 hours after return to normoxia. Up-regulated in ischemic brain. Belongs to the RAMP4 family.